Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Hard Disk shopping experience:

1. Compare - without doubt the biggest advantage that the Hard Disk offers shoppers today is the ability to compare thousands of Hard Disk at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.

2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about

3. Testimonials - don't know anybody that has bought a Hard Disk? Wrong! If the Hard Disk is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.

4. Questions - Got a question about Hard Disk then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....

5. Reputation - Never heard of the company selling Hard Disk? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Hard Disk and build up a picture of their reputation for sales, returns, customer service, delivery etc.

6. Returns - still worried that even after all of the above your Hard Disk wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.

7. Feedback - happy with your Hard Disk then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.

8. Security - check for the yellow padlock on the Hard Disk site before you buy, and the s after http:/ /i.e. https:// = a secure site

9. Contact - got a question about Hard Disk, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.

10. Payment - ready to pay for your Hard Disk, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.

{{ infobox computer hardware generic| name = Hard disk drive| image = Hard disk platter reflection.jpg| caption = An IBM hard disk drive with the metal cover removed. The platters are highly reflective.| invent-date = September 13 1956 team led by [Reynold Johnson (on [personal computer often integrated into motherboard)] (IDE) interface| via1_2 = SATA interface] interface| via1_4 = SCSI interface (popular on Server (computing))| via1_5 = Fibre Channel interface (almost exclusively found on servers)| class-name = Market segment| class1 = Desktop computer| class2 = Mobile computing| class3 = Enterprise| class4 = Consumer| class5 = Other/Miscellaneous-->

A hard disk drive (HDD), commonly referred to as a hard drive, hard disk or fixed disk drive,Other terms used to describe hard disk drives include disk drive, disk file, DASD (Direct Access Storage Device), fixed disk, CKD disk and Winchester Disk Drive (after the Early IBM disk storage). is a non-volatile storage device which stores digitally encoded data on rapidly rotating hard disk platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit with fixed media. How Hard Disks Work, howstuffworks.com

HDDs were originally developed for use with computers. In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders, digital audio players, personal digital assistants, digital cameras, and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung Electronics and Nokia. Finally! The Samsung SPH-V5400, world's first cellphone with a hard drive, engadget.com, 6 September 2004 The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as redundant array of independent disks arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

Technology HDDs record data by magnetizing a ferromagnetism material in a pattern that represents the data. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called Hard disk platter, onto which the data is recorded. The platters are made from a non-magnetic material, usually glass or aluminum, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.

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The platters are spun at very high speeds. Information is written to a platter as it rotates past mechanisms called Disk read-and-write heads that operate very close over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator.

The magnetic surface of each platter is divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early HDDs used an electromagnet both to generate this field and to read the data by using electromagnetic induction. Later versions of inductive heads included metal in Gap (MIG) heads and thin film heads. In today's heads, the read and write elements are separate but in close proximity on the head portion of an actuator arm. The read element is typically giant magnetoresistance while the write element is typically thin-film inductive.http://www.hitachigst.com/hdd/technolo/gmr/gmr.htm

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.Brian Hayes, Terabyte Territory, American Scientist, Vol 90 No 3 (May-June 2002) p. 212 Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, which has been used in some hard drives as of 2006.

Hard disk drives are sealed to prevent dust and other sources of contamination from interfering with the operation of the hard disks heads. The hard drives are not air tight, but rather utilize an extremely fine air filter, to allow for air inside the hard drive enclosure. The spinning of the disks causes the air to circulate forcing any particulates to become trapped on the filter. The same air currents also act as a gas bearing which enables the heads to float on an air cushions above the surfaces of the disks.

Hard drives are precise devices, moving at very high speed, and a number of analogy have been made to try to describe this. One states:

Capacity and access speed hard disk drive capacity (in gigabytes). The plot is logarithmic scale, so the fit line corresponds to exponential growth.

Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk. Consequently, hard disk drives can store much more data than floppy disk drives and access and transmit it faster. In 2007, a typical enterprise, i.e. workstation HDD might store between 160 Gigabyte and 1 Terabyte of data (as of local US market by July 2007), rotate at 7,200 or 10,000 revolutions per minute (RPM), and have a sequential media transfer rate of over 80 MB/s. The fastest enterprise HDDs spin at 15,000 rpm, and can achieve sequential media transfer speeds up to and beyond 110 MB/s.http://anandtech.com/storage/showdoc.aspx?i=2922&p=1 Review of Western Digital's fastest HDD as of February 2007 Mobile, i.e., Laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. In the 1990s, most spun at 4,200 rpm.The 1999 Disk/Trend Hard Disk Drive Report lists 81 mobile HDDs having rotational speeds ranging from 3,634 to 4,900 rpm with 44 models operating at 4200 rpm In 2007, a typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium.

The exponential increases in disk space and data access speeds of HDDs have enabled the commercial viability of consumer products that require large storage capacities, such as the TiVo personal video recorder and digital music players. In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based systems with extraordinary capacity requirements, such as the search and email systems offered by companies like Google.

The main way to decrease access time is to increase rotational speed, while the main way to increase throughput and storage capacity is to increase areal density. A vice president of Seagate Technology projects a future growth in disk density of 40% per year.http://www.hardwarezone.com/articles/view.php?cid=1&id=1805&pg=2 Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

As of 2006, disk drives include perpendicular recording technology, in the attempt to enhance recording density and throughput. 500GB SATA drives reviews

The first 3.5" HDD marketed as able to store 1 TB is the Hitachi Deskstar 7K1000. The drive contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space. Hitachi's 7K1000 Terabyte Hard Drive Hitachi has since been joined by Samsung and Seagate in the 1 TB drive market. Seagate, Samsung Begin to Ship 1 TB Desktop Hard Drives

{]| 146 Millimeter| 47 gigabyteSeagate Elite 47, shipped 12/97 per 1998 Disk/Trend Report - Rigid Disk Drives| 14|-| 5.25" Drive bay#Half-height| 146 mm| 19.3 gigabyteQuantum Bigfoot TS, shipped 10/98 per 1999 Disk/Trend Report - Rigid Disk Drives| 4The Quantum Bigfoot TS used a maximum of 3 platters, other earlier and lower capacity product used up to 4 platters in a 5.25" HH form factor, e.g. Microscience HH1090 circa 1989.|-| 3.5"| 102 mm| 1.2 terabyte| 5|-| 2.5"| 69.9 mm| 320 GB 070824 http://www.engadget.com| 3|-| 1.8" (PCMCIA)])| 53.8 mm|}

Capacity measurements

The capacity of an HDD can be calculated by multiplying the number of cylinder (computer) by the number of heads by the number of sectors by the number of bytes/sector (most commonly 512). On AT Attachment drives bigger than 8 gigabytes, the values are set to 16383 cylinder, 16 heads, 63 sectors for compatibility with older operating systems. It should be noted that the values for cylinder, head & sector reported by a modern drive are not the actual physical parameters since, amongst other things, with zone bit recording the number of sectors varies by zone.

Hard disk drive manufacturers specify disk capacity using the SI prefixes mega, giga, and tera and their abbreviations M, G and T, respectively. Byte is typically abbreviated B.

Operating systems frequently report capacity using the same abbreviations but with a binary interpretation. For instance, the prefix mega can also mean 220 (1,048,576), which is approximately 1,000,000. Similar usage has been applied to prefixes of greater magnitude. This results in a discrepancy between the disk manufacturer's stated capacity and what the system reports. The difference becomes much more noticeable in the multi-gigabyte range. For example, Microsoft Windows reports disk capacity both in decimal to 12 or more significant digits and with binary prefixes to 3 significant digits. Thus a disk specified by a disk manufacturer as a 30 GB disk might have its capacity reported by Windows 2000 both as "30,065,098,568 bytes" and "28.0 GB" The disk manufacturer used the SI definition of "giga", 109 to arrive at 30 GB; however, because the utilities provided by Windows define a gigabyte as 1,073,741,824 bytes (230 bytes, properly known as gibibyte, or GiB), the operating system reports capacity of the disk drive as 28.0 GB.

Hard disk drive characteristics

Capacity of a hard disk drive is usually quoted in gigabytes. Older HDDs quoted their smaller capacities in megabytes.

The data transfer rate at the inner zone ranges from 44.2 MB/s to 74.5 MB/s, while the transfer rate at the outer zone ranges from 74.0 MB/s to 111.4 MB/s. An HDD's random access time ranges from 5 ms to 15 ms.

The physical size of a hard disk drive is quoted in inches. The majority of HDDs used in desktops today are wide, while the majority of those used in laptops are wide. As of early 2007, manufacturers have started selling SATA and SAS 2.5 inch drives for use in servers and desktops.

An increasingly common form factor is the ATA-7 LIF form factor used inside digital audio players and subnotebooks, which provide up to 160GB storage capacity at low power consumption and are highly shock-resistant. A previous HDD standard exists, for 2–5 GB sized disks that fit directly into a PC card expansion slot. From these, the smaller form factor was evolved, which is designed to fit the dimensions of Compact Flash Type II, which is also usually used as storage for portable devices including digital cameras. 1 inch was a de facto form factor led by IBM's Microdrive, but is now generically called 1 inch due to other manufacturers producing similar products. There is also a 0.85 inch form factor produced by Toshiba for use in mobile phones and similar applications, including Secure Digital card/MultiMediaCard slot compatible HDDs optimized for video storage on 4G handsets.

The size designations are more nomenclature than descriptive. The names refer to the width of the disk inserted into the drive rather than the actual width of the entire drive. A drive has an actual width of , a drive , a drive . A drive can have different widths, depending on its form factor. A PCMCIA drive has a width of 54 mm, while an ATA-7 LIF form factor drive has a width of 53.85 mm.

A hard disk is defined to be at "full height" if its height is . It is "half height" at a height of . A "slim height" or "low profile" HDD has a height of . "Ultra low profile" drives can have heights of , , or .

Access and interfaces Hard disk drives are accessed over one of a number of bus types, including parallel Advanced Technology Attachment (also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to busses that they cannot communicate with natively, such as IEEE 1394 and Universal Serial Bus.

Back in the days of the ST-506 interface, the data encoder scheme was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5 megabits per second. Later on, controllers using 2,7 Run Length Limited (or just "RLL") encoding increased the transfer rate by fifty percent, to 7.5 megabits per second; it also increased disk capacity by fifty percent.

Many ST-506 interface disk drives were only specified by the manufacturer to run at the lower MFM data rate, while other models (usually more expensive versions of the same basic disk drive) were specified to run at the higher RLL data rate. In some cases, a disk drive had sufficient margin to allow the MFM specified model to run at the faster RLL data rate; however, this was often unreliable and was not recommended. (An RLL-certified disk drive could run on a MFM controller, but with 1/3 less data capacity and speed.)

Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives weren't downward compatible (i.e. a 15 or 20 megabit disk drive wouldn't run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.

SCSI originally had just one speed, 5 MHz (for a maximum data rate of 5 megabytes per second), but later this was increased dramatically. The SCSI bus speed had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drive's internal data bus; however, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave (just like ST-506 disks) when used on slow computers, such as early IBM PC compatibles and early Apple Macintoshes.

ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and couldn't run in a master/slave setup (two disks on the same cable). This was mostly remedied by the mid-1990s, when ATA's specification was standardised and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA devices).

Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.

FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/IEEE 1394 models are able to Daisy chain#Computer Engineering in order to continue adding peripherals without requiring additional ports on the computer itself.

Disk interface families used in personal computers Notable families of disk interfaces include:

{| class="wikitable"|-! Acronym !! Meaning !! Description]||Shugart Associates System Interface ||Historical predecessor to SCSI.|-|SCSI oriented that handles [concurrent operations.]||Serial Attached SCSI||Improvement of SCSI, uses serial communication instead parallel.|-|ST-506|| ||Historical Seagate interface (minor improvement over ST-506).|-|[Enhanced Small Disk Interface||Enhanced Small Disk Interface ||Historical; backwards compatible with ST-412/506, but faster and more integrated.|-|Advanced Technology Attachment||Advanced Technology Attachment ||Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations.]||Serial ATA||Improvement of ATA, uses serial communication instead parallel.|}

Integrity

Due to the extremely close spacing between the heads and the disk surface, any contamination of the read-write heads or platters can lead to a head crash — a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, wear and tear, corrosion, or poorly manufactured platters and heads.

The HDD's spindle system relies on air pressure inside the enclosure to support the heads at their proper flying height while the disk rotates. An HDD requires a certain range of air pressures in order to operate properly. The connection to the external environment and pressure occurs through a small hole in the enclosure (about 0.5 mm in diameter), usually with a carbon filter on the inside (the breather filter, see below). If the air pressure is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 10,000 feet (3,000 m). This does not apply to pressurized enclosures, like an aircraft pressurized cabin. Modern disks include temperature sensors and adjust their operation to the operating environment.Breather holes can be seen on all disks — they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating disk is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation (or "recirc") filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation.Very high humidity for extended periods can corrode the heads and platters.

For giant magnetoresistive effect (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal asperity," a problem which can partially be dealt with by proper electronic filtering of the read signal).

The hard disk's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.

Landing zones of a hard disk head. The size of the front face (which is the "trailing face" of the slider) is about 0.3 mm × 1.0 mm. The (not visible) bottom face of the slider is about 1.0 mm × 1.25 mm (so called "nano" size) and faces the platter. One functional part of the head is the round, orange structure in the middle - the photolithography defined copper coil of the write transducer. Also note the electric connections by wires bonded to gold-plated pads.

In old disk models, sudden power interruptions or a power supply failure sometimes resulted in the device shutting down with the heads in the data zone, which greatly increased the risk of data loss. In fact, a manual procedure existed for parking the hard disk heads before shutting down the computer.

To prevent such situation, most modern HDDs, when powering down, move the heads to a landing zone, an area of the platter usually near its inner diameter (ID), where no data is stored. This area is called the Contact Start/Stop (CSS) zone. Disks are designed such that either a spring or, more recently, rotational inertia in the platters is used to safely park the heads in the case of unexpected power loss.

Spring tension from the head mounting constantly pushes the heads towards the platter. While the disk is spinning, the heads are supported by an air bearing and experience no physical contact or wear. In CSS drives the sliders carrying the head sensors (often also just called heads) are designed to reliably survive a number of landings and takeoffs from the media surface, though wear and tear on these microscopic components eventually takes its toll. Most manufacturers design the sliders to survive 50,000 contact cycles before the chance of damage on startup rises above 50%. However, the decay rate is not linear—when a disk is younger and has fewer start-stop cycles, it has a better chance of surviving the next startup than an older, higher-mileage disk (as the head literally drags along the disk's surface until the air bearing is established). For example, the Seagate Barracuda 7200.10 series of desktop hard disks are rated to 50,000 start-stop cycles.http://www.seagate.com/support/disc/manuals/sata/100402371a.pdf This means that no failures attributed to the head-platter interface were seen before at least 50,000 start-stop cycles during testing.

Around 1995 IBM pioneered a technology where a landing zone on the disk is made by a precision laser process (Laser Zone Texture = LZT) producing an array of smooth nanometer-scale "bumps" in a landing zone, thus vastly improving stiction and wear performance. This technology is still largely in use today (2006). In most mobile applications, the heads are lifted off the platters onto plastic "ramps" near the outer disk edge, thus eliminating the risks of wear and stiction altogether and greatly improving non-operating shock performance. All HDDs use one of these two technologies. Each has a list of advantages and drawbacks in terms of loss of storage space, relative difficulty of mechanical tolerance control, cost of implementation, etc.

IBM created a technology for their ThinkPad line of laptop computers called the Active Protection System. When a sudden, sharp movement is detected by the built-in accelerometer in the Thinkpad, internal hard disk heads automatically unload themselves into the parking zone to reduce the risk of any potential data loss or scratches made. Apple Inc. later also utilized this technology in their PowerBook, iBook, MacBook Pro, and MacBook line, known as the Sudden Motion Sensor. Toshiba has released similar technology in their laptops. Toshiba HDD Protection measures.

With CSS technology, increased humidity in addition to causing corrosion, can also lead to increased stiction (the tendency for the heads to stick to the platter surface). This can cause physical damage to the platter motor or spindle motor.

===Disk failures and their metrics=== Most major hard disk and motherboard vendors now support Self-Monitoring, Analysis, and Reporting Technology (S.M.A.R.T.), which attempt to alert users to impending failures.

However, not all failures are predictable. Normal use eventually can lead to a breakdown in the inherently fragile device, which makes it essential for the user to periodically back up the data onto a separate storage device. Failure to do so can lead to the loss of data. While it may be possible to recover lost information, it is normally an extremely costly procedure, and it is not possible to guarantee success in the attempt. A 2007 study published by Google suggested very little correlation between failure rates and either high temperature or activity level.Barroso, L.A., et al. Failure Trends in a Large Disk Drive Population. February 2007. While several S.M.A.R.T. parameters have an impact on failure probability, a large fraction of failed drives do not produce predictive S.M.A.R.T. parameters. S.M.A.R.T. parameters alone may not be useful for predicting individual drive failures.

SCSI, SAS and FC drives are typically more expensive, as they are traditionally used in servers and disk arrays. Inexpensive ATA and SATA drives evolved in the home computer market, hence the general opinion is that they are less reliable. As those two worlds started to overlap, reliability subject became somewhat controversial. It is worth to note, that generally a disk drive has a low failure rate because of increased quality of heads, platters and supporting manufacturing processes, not just because of having certain interface.

The mean time to failure (MTBF) of SATA drives is usually about 600,000 hours (some drives such as Western Digital Raptor have rated 1.2 million hours MTBF), while SCSI drives are rated for upwards of 1,500,000 hours. However, independent research done on hard drives reliability have indicated MTBF is not a reliable estimate of a drive's longevity. MTBF is conducted in laboratory environments in test chambers and is an important metric to determine the quality of a disk drive prior to entering high volume production. Once the drive product is in production, the more valid metric is annualized failure rate (AFR). AFR is the percentage of real-world drive failures after shipping.

SAS drives are comparable to SCSI drives, with high MTBF and high reliability.

Enterprise SATA drives designed and produced for enterprise markets, unlike standard SATA drives, have reliability comparable other enterprise class drives.

Typically enterprise drives (all enterprise drives, including SCSI, SAS, enterprise SATA and FC) experience between .70%-.78% annual failure rates from the total installed drives.

Manufacturers 3.5 inch 40 GB HDD.

The technological resources and know-how required for modern drive development and production mean that as of 2007, over 98% of the world's HDDs are manufactured by just a handful of large firms: Seagate Technology (which now owns Maxtor), Western Digital, Samsung Electronics, and Hitachi Ltd. (which owns the former disk manufacturing division of International Business Machines). Fujitsu continues to make mobile- and server-class disks but exited the desktop-class market in 2001. Toshiba is a major manufacturer of 2.5-inch and 1.8-inch notebook disks. ExcelStor is a small HDD manufacturer.

Dozens of former HDD manufacturers have gone out of business, merged, or closed their HDD divisions; as capacities and demand for products increased, profits became hard to find, and the market underwent significant consolidation in the late 1980s and late 1990s. The first notable casualty of the business in the PC era was Computer Memories Inc. or CMI; after an incident with faulty 20 MB AT disks in 1985,Apparently the CMI disks suffered from a higher soft error rate than IBM's other suppliers (Seagate and MiniScribe) but the bugs in Microsoft's DOS Operating system may have turned these recoverable errors into hard failures. At some point, possibly MS-DOS 3.0, soft errors were reported as disk hard errors and a subsequent Microsoft patch turned soft errors into corrupted memory with unpredictable results ("crashes"). MS-DOS 3.3 apparently resolved this series of problems but by that time it was too late for CMI. See also, "IBM and CMI in Joint Effort to Rehab AT Hard-Disk Rejects," PC Week, v.2 n.11, p.1, March 19, 1985 CMI's reputation never recovered, and they exited the HDD business in 1987. Another notable failure was MiniScribe, who went bankrupt in 1990 after it was found that they had engaged in accounting fraud and inflated sales numbers for several years. Many other smaller companies (like Kalok, Microscience International Corporation, LaPine, Areal, Priam and PrairieTek) also did not survive the shakeout, and had disappeared by 1993; Micropolis Corporation was able to hold on until 1997, and JT Storage, a relative latecomer to the scene, lasted only a few years and was gone by 1999, after attempting to manufacture HDDs in India. Their claim to fame was creating a new 3" form factor drive for use in laptops. Quantum and Integral also invested in the 3" form factor; but eventually gave up as this form factor failed to catch on. Rodime plc was also an important manufacturer during the 1980s, but stopped making disks in the early 1990s amid the shakeout and now concentrates on technology licensing; they hold a number of patents related to 3.5-inch form factor HDDs.



History

For many years, HDDs were large, cumbersome devices, more suited to use in the protected environment of a data center or large office than in a harsh industrial environment (due to their delicacy), or small office or home (due to their size and power consumption). Before the early 1980s, most HDDs had 8-inch (20 cm) or 14-inch (35 cm) platters, required an equipment rack or a large amount of floor space (especially the large removable-media disks, which were often referred to as "washing machines"), and in many cases needed high-current or even three-phase power hookups due to the large motors they used. Because of this, HDDs were not commonly used with microcomputers until after 1980, when Seagate Technology introduced the ST-506, the first 5.25-inch HDD, with a capacity of 5 megabytes. In fact, in its factory configuration, the original IBM PC (IBM 5150) was not equipped with a hard disk drive.

Most microcomputer HDDs in the early 1980s were not sold under their manufacturer's names, but by Original equipment manufacturers as part of larger peripherals (such as the Corvus Disk System and the Apple ProFile). The IBM PC/XT had an internal HDD, however, and this started a trend toward buying "bare" disks (often by mail order) and installing them directly into a system. Hard disk drive makers started marketing to end users as well as OEMs, and by the mid-1990s, HDDs had become available on retail store shelves.

While internal disks became the system of choice on PCs, external HDDs remained popular for much longer on the Apple Macintosh and other platforms. The first Apple Macintosh built between 1984 and 1986 had a closed architecture that did not support an external or internal hard drive. In 1986, Apple added a SCSI port on the back, making external expansion easy. External SCSI drives were also popular with older microcomputers such as the Apple II series, and were also used extensively in server (computing)s, a usage which is still popular today. The appearance in the late 1990s of high-speed external interfaces such as Universal Serial Bus and FireWire has made external disk systems popular among PC users once again, especially for laptop users, users that install Linux in the additional external unit and users who move large amounts of data between two or more areas. Most HDD makers now make their disks available in external cases.

See also

Notes and references External links

{{ infobox computer hardware generic| name = Hard disk drive| image = Hard disk platter reflection.jpg| caption = An IBM hard disk drive with the metal cover removed. The platters are highly reflective.| invent-date = September 13 1956 team led by [Reynold Johnson (on [personal computer often integrated into motherboard)] (IDE) interface| via1_2 = SATA interface] interface| via1_4 = SCSI interface (popular on Server (computing))| via1_5 = Fibre Channel interface (almost exclusively found on servers)| class-name = Market segment| class1 = Desktop computer| class2 = Mobile computing| class3 = Enterprise| class4 = Consumer| class5 = Other/Miscellaneous-->

A hard disk drive (HDD), commonly referred to as a hard drive, hard disk or fixed disk drive,Other terms used to describe hard disk drives include disk drive, disk file, DASD (Direct Access Storage Device), fixed disk, CKD disk and Winchester Disk Drive (after the Early IBM disk storage). is a non-volatile storage device which stores digitally encoded data on rapidly rotating hard disk platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit with fixed media. How Hard Disks Work, howstuffworks.com

HDDs were originally developed for use with computers. In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders, digital audio players, personal digital assistants, digital cameras, and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung Electronics and Nokia. Finally! The Samsung SPH-V5400, world's first cellphone with a hard drive, engadget.com, 6 September 2004 The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as redundant array of independent disks arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

Technology HDDs record data by magnetizing a ferromagnetism material in a pattern that represents the data. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called Hard disk platter, onto which the data is recorded. The platters are made from a non-magnetic material, usually glass or aluminum, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.

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The platters are spun at very high speeds. Information is written to a platter as it rotates past mechanisms called Disk read-and-write heads that operate very close over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator.

The magnetic surface of each platter is divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early HDDs used an electromagnet both to generate this field and to read the data by using electromagnetic induction. Later versions of inductive heads included metal in Gap (MIG) heads and thin film heads. In today's heads, the read and write elements are separate but in close proximity on the head portion of an actuator arm. The read element is typically giant magnetoresistance while the write element is typically thin-film inductive.http://www.hitachigst.com/hdd/technolo/gmr/gmr.htm

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.Brian Hayes, Terabyte Territory, American Scientist, Vol 90 No 3 (May-June 2002) p. 212 Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, which has been used in some hard drives as of 2006.

Hard disk drives are sealed to prevent dust and other sources of contamination from interfering with the operation of the hard disks heads. The hard drives are not air tight, but rather utilize an extremely fine air filter, to allow for air inside the hard drive enclosure. The spinning of the disks causes the air to circulate forcing any particulates to become trapped on the filter. The same air currents also act as a gas bearing which enables the heads to float on an air cushions above the surfaces of the disks.

Hard drives are precise devices, moving at very high speed, and a number of analogy have been made to try to describe this. One states:

Capacity and access speed hard disk drive capacity (in gigabytes). The plot is logarithmic scale, so the fit line corresponds to exponential growth.

Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk. Consequently, hard disk drives can store much more data than floppy disk drives and access and transmit it faster. In 2007, a typical enterprise, i.e. workstation HDD might store between 160 Gigabyte and 1 Terabyte of data (as of local US market by July 2007), rotate at 7,200 or 10,000 revolutions per minute (RPM), and have a sequential media transfer rate of over 80 MB/s. The fastest enterprise HDDs spin at 15,000 rpm, and can achieve sequential media transfer speeds up to and beyond 110 MB/s.http://anandtech.com/storage/showdoc.aspx?i=2922&p=1 Review of Western Digital's fastest HDD as of February 2007 Mobile, i.e., Laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. In the 1990s, most spun at 4,200 rpm.The 1999 Disk/Trend Hard Disk Drive Report lists 81 mobile HDDs having rotational speeds ranging from 3,634 to 4,900 rpm with 44 models operating at 4200 rpm In 2007, a typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium.

The exponential increases in disk space and data access speeds of HDDs have enabled the commercial viability of consumer products that require large storage capacities, such as the TiVo personal video recorder and digital music players. In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based systems with extraordinary capacity requirements, such as the search and email systems offered by companies like Google.

The main way to decrease access time is to increase rotational speed, while the main way to increase throughput and storage capacity is to increase areal density. A vice president of Seagate Technology projects a future growth in disk density of 40% per year.http://www.hardwarezone.com/articles/view.php?cid=1&id=1805&pg=2 Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

As of 2006, disk drives include perpendicular recording technology, in the attempt to enhance recording density and throughput. 500GB SATA drives reviews

The first 3.5" HDD marketed as able to store 1 TB is the Hitachi Deskstar 7K1000. The drive contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space. Hitachi's 7K1000 Terabyte Hard Drive Hitachi has since been joined by Samsung and Seagate in the 1 TB drive market. Seagate, Samsung Begin to Ship 1 TB Desktop Hard Drives

{]| 146 Millimeter| 47 gigabyteSeagate Elite 47, shipped 12/97 per 1998 Disk/Trend Report - Rigid Disk Drives| 14|-| 5.25" Drive bay#Half-height| 146 mm| 19.3 gigabyteQuantum Bigfoot TS, shipped 10/98 per 1999 Disk/Trend Report - Rigid Disk Drives| 4The Quantum Bigfoot TS used a maximum of 3 platters, other earlier and lower capacity product used up to 4 platters in a 5.25" HH form factor, e.g. Microscience HH1090 circa 1989.|-| 3.5"| 102 mm| 1.2 terabyte| 5|-| 2.5"| 69.9 mm| 320 GB 070824 http://www.engadget.com| 3|-| 1.8" (PCMCIA)])| 53.8 mm|}

Capacity measurements

The capacity of an HDD can be calculated by multiplying the number of cylinder (computer) by the number of heads by the number of sectors by the number of bytes/sector (most commonly 512). On AT Attachment drives bigger than 8 gigabytes, the values are set to 16383 cylinder, 16 heads, 63 sectors for compatibility with older operating systems. It should be noted that the values for cylinder, head & sector reported by a modern drive are not the actual physical parameters since, amongst other things, with zone bit recording the number of sectors varies by zone.

Hard disk drive manufacturers specify disk capacity using the SI prefixes mega, giga, and tera and their abbreviations M, G and T, respectively. Byte is typically abbreviated B.

Operating systems frequently report capacity using the same abbreviations but with a binary interpretation. For instance, the prefix mega can also mean 220 (1,048,576), which is approximately 1,000,000. Similar usage has been applied to prefixes of greater magnitude. This results in a discrepancy between the disk manufacturer's stated capacity and what the system reports. The difference becomes much more noticeable in the multi-gigabyte range. For example, Microsoft Windows reports disk capacity both in decimal to 12 or more significant digits and with binary prefixes to 3 significant digits. Thus a disk specified by a disk manufacturer as a 30 GB disk might have its capacity reported by Windows 2000 both as "30,065,098,568 bytes" and "28.0 GB" The disk manufacturer used the SI definition of "giga", 109 to arrive at 30 GB; however, because the utilities provided by Windows define a gigabyte as 1,073,741,824 bytes (230 bytes, properly known as gibibyte, or GiB), the operating system reports capacity of the disk drive as 28.0 GB.

Hard disk drive characteristics

Capacity of a hard disk drive is usually quoted in gigabytes. Older HDDs quoted their smaller capacities in megabytes.

The data transfer rate at the inner zone ranges from 44.2 MB/s to 74.5 MB/s, while the transfer rate at the outer zone ranges from 74.0 MB/s to 111.4 MB/s. An HDD's random access time ranges from 5 ms to 15 ms.

The physical size of a hard disk drive is quoted in inches. The majority of HDDs used in desktops today are wide, while the majority of those used in laptops are wide. As of early 2007, manufacturers have started selling SATA and SAS 2.5 inch drives for use in servers and desktops.

An increasingly common form factor is the ATA-7 LIF form factor used inside digital audio players and subnotebooks, which provide up to 160GB storage capacity at low power consumption and are highly shock-resistant. A previous HDD standard exists, for 2–5 GB sized disks that fit directly into a PC card expansion slot. From these, the smaller form factor was evolved, which is designed to fit the dimensions of Compact Flash Type II, which is also usually used as storage for portable devices including digital cameras. 1 inch was a de facto form factor led by IBM's Microdrive, but is now generically called 1 inch due to other manufacturers producing similar products. There is also a 0.85 inch form factor produced by Toshiba for use in mobile phones and similar applications, including Secure Digital card/MultiMediaCard slot compatible HDDs optimized for video storage on 4G handsets.

The size designations are more nomenclature than descriptive. The names refer to the width of the disk inserted into the drive rather than the actual width of the entire drive. A drive has an actual width of , a drive , a drive . A drive can have different widths, depending on its form factor. A PCMCIA drive has a width of 54 mm, while an ATA-7 LIF form factor drive has a width of 53.85 mm.

A hard disk is defined to be at "full height" if its height is . It is "half height" at a height of . A "slim height" or "low profile" HDD has a height of . "Ultra low profile" drives can have heights of , , or .

Access and interfaces Hard disk drives are accessed over one of a number of bus types, including parallel Advanced Technology Attachment (also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to busses that they cannot communicate with natively, such as IEEE 1394 and Universal Serial Bus.

Back in the days of the ST-506 interface, the data encoder scheme was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5 megabits per second. Later on, controllers using 2,7 Run Length Limited (or just "RLL") encoding increased the transfer rate by fifty percent, to 7.5 megabits per second; it also increased disk capacity by fifty percent.

Many ST-506 interface disk drives were only specified by the manufacturer to run at the lower MFM data rate, while other models (usually more expensive versions of the same basic disk drive) were specified to run at the higher RLL data rate. In some cases, a disk drive had sufficient margin to allow the MFM specified model to run at the faster RLL data rate; however, this was often unreliable and was not recommended. (An RLL-certified disk drive could run on a MFM controller, but with 1/3 less data capacity and speed.)

Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives weren't downward compatible (i.e. a 15 or 20 megabit disk drive wouldn't run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.

SCSI originally had just one speed, 5 MHz (for a maximum data rate of 5 megabytes per second), but later this was increased dramatically. The SCSI bus speed had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drive's internal data bus; however, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave (just like ST-506 disks) when used on slow computers, such as early IBM PC compatibles and early Apple Macintoshes.

ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and couldn't run in a master/slave setup (two disks on the same cable). This was mostly remedied by the mid-1990s, when ATA's specification was standardised and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA devices).

Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.

FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/IEEE 1394 models are able to Daisy chain#Computer Engineering in order to continue adding peripherals without requiring additional ports on the computer itself.

Disk interface families used in personal computers Notable families of disk interfaces include:

{| class="wikitable"|-! Acronym !! Meaning !! Description]||Shugart Associates System Interface ||Historical predecessor to SCSI.|-|SCSI oriented that handles [concurrent operations.]||Serial Attached SCSI||Improvement of SCSI, uses serial communication instead parallel.|-|ST-506|| ||Historical Seagate interface (minor improvement over ST-506).|-|[Enhanced Small Disk Interface||Enhanced Small Disk Interface ||Historical; backwards compatible with ST-412/506, but faster and more integrated.|-|Advanced Technology Attachment||Advanced Technology Attachment ||Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations.]||Serial ATA||Improvement of ATA, uses serial communication instead parallel.|}

Integrity

Due to the extremely close spacing between the heads and the disk surface, any contamination of the read-write heads or platters can lead to a head crash — a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, wear and tear, corrosion, or poorly manufactured platters and heads.

The HDD's spindle system relies on air pressure inside the enclosure to support the heads at their proper flying height while the disk rotates. An HDD requires a certain range of air pressures in order to operate properly. The connection to the external environment and pressure occurs through a small hole in the enclosure (about 0.5 mm in diameter), usually with a carbon filter on the inside (the breather filter, see below). If the air pressure is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 10,000 feet (3,000 m). This does not apply to pressurized enclosures, like an aircraft pressurized cabin. Modern disks include temperature sensors and adjust their operation to the operating environment.Breather holes can be seen on all disks — they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating disk is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation (or "recirc") filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation.Very high humidity for extended periods can corrode the heads and platters.

For giant magnetoresistive effect (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal asperity," a problem which can partially be dealt with by proper electronic filtering of the read signal).

The hard disk's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.

Landing zones of a hard disk head. The size of the front face (which is the "trailing face" of the slider) is about 0.3 mm × 1.0 mm. The (not visible) bottom face of the slider is about 1.0 mm × 1.25 mm (so called "nano" size) and faces the platter. One functional part of the head is the round, orange structure in the middle - the photolithography defined copper coil of the write transducer. Also note the electric connections by wires bonded to gold-plated pads.

In old disk models, sudden power interruptions or a power supply failure sometimes resulted in the device shutting down with the heads in the data zone, which greatly increased the risk of data loss. In fact, a manual procedure existed for parking the hard disk heads before shutting down the computer.

To prevent such situation, most modern HDDs, when powering down, move the heads to a landing zone, an area of the platter usually near its inner diameter (ID), where no data is stored. This area is called the Contact Start/Stop (CSS) zone. Disks are designed such that either a spring or, more recently, rotational inertia in the platters is used to safely park the heads in the case of unexpected power loss.

Spring tension from the head mounting constantly pushes the heads towards the platter. While the disk is spinning, the heads are supported by an air bearing and experience no physical contact or wear. In CSS drives the sliders carrying the head sensors (often also just called heads) are designed to reliably survive a number of landings and takeoffs from the media surface, though wear and tear on these microscopic components eventually takes its toll. Most manufacturers design the sliders to survive 50,000 contact cycles before the chance of damage on startup rises above 50%. However, the decay rate is not linear—when a disk is younger and has fewer start-stop cycles, it has a better chance of surviving the next startup than an older, higher-mileage disk (as the head literally drags along the disk's surface until the air bearing is established). For example, the Seagate Barracuda 7200.10 series of desktop hard disks are rated to 50,000 start-stop cycles.http://www.seagate.com/support/disc/manuals/sata/100402371a.pdf This means that no failures attributed to the head-platter interface were seen before at least 50,000 start-stop cycles during testing.

Around 1995 IBM pioneered a technology where a landing zone on the disk is made by a precision laser process (Laser Zone Texture = LZT) producing an array of smooth nanometer-scale "bumps" in a landing zone, thus vastly improving stiction and wear performance. This technology is still largely in use today (2006). In most mobile applications, the heads are lifted off the platters onto plastic "ramps" near the outer disk edge, thus eliminating the risks of wear and stiction altogether and greatly improving non-operating shock performance. All HDDs use one of these two technologies. Each has a list of advantages and drawbacks in terms of loss of storage space, relative difficulty of mechanical tolerance control, cost of implementation, etc.

IBM created a technology for their ThinkPad line of laptop computers called the Active Protection System. When a sudden, sharp movement is detected by the built-in accelerometer in the Thinkpad, internal hard disk heads automatically unload themselves into the parking zone to reduce the risk of any potential data loss or scratches made. Apple Inc. later also utilized this technology in their PowerBook, iBook, MacBook Pro, and MacBook line, known as the Sudden Motion Sensor. Toshiba has released similar technology in their laptops. Toshiba HDD Protection measures.

With CSS technology, increased humidity in addition to causing corrosion, can also lead to increased stiction (the tendency for the heads to stick to the platter surface). This can cause physical damage to the platter motor or spindle motor.

===Disk failures and their metrics=== Most major hard disk and motherboard vendors now support Self-Monitoring, Analysis, and Reporting Technology (S.M.A.R.T.), which attempt to alert users to impending failures.

However, not all failures are predictable. Normal use eventually can lead to a breakdown in the inherently fragile device, which makes it essential for the user to periodically back up the data onto a separate storage device. Failure to do so can lead to the loss of data. While it may be possible to recover lost information, it is normally an extremely costly procedure, and it is not possible to guarantee success in the attempt. A 2007 study published by Google suggested very little correlation between failure rates and either high temperature or activity level.Barroso, L.A., et al. Failure Trends in a Large Disk Drive Population. February 2007. While several S.M.A.R.T. parameters have an impact on failure probability, a large fraction of failed drives do not produce predictive S.M.A.R.T. parameters. S.M.A.R.T. parameters alone may not be useful for predicting individual drive failures.

SCSI, SAS and FC drives are typically more expensive, as they are traditionally used in servers and disk arrays. Inexpensive ATA and SATA drives evolved in the home computer market, hence the general opinion is that they are less reliable. As those two worlds started to overlap, reliability subject became somewhat controversial. It is worth to note, that generally a disk drive has a low failure rate because of increased quality of heads, platters and supporting manufacturing processes, not just because of having certain interface.

The mean time to failure (MTBF) of SATA drives is usually about 600,000 hours (some drives such as Western Digital Raptor have rated 1.2 million hours MTBF), while SCSI drives are rated for upwards of 1,500,000 hours. However, independent research done on hard drives reliability have indicated MTBF is not a reliable estimate of a drive's longevity. MTBF is conducted in laboratory environments in test chambers and is an important metric to determine the quality of a disk drive prior to entering high volume production. Once the drive product is in production, the more valid metric is annualized failure rate (AFR). AFR is the percentage of real-world drive failures after shipping.

SAS drives are comparable to SCSI drives, with high MTBF and high reliability.

Enterprise SATA drives designed and produced for enterprise markets, unlike standard SATA drives, have reliability comparable other enterprise class drives.

Typically enterprise drives (all enterprise drives, including SCSI, SAS, enterprise SATA and FC) experience between .70%-.78% annual failure rates from the total installed drives.

Manufacturers 3.5 inch 40 GB HDD.

The technological resources and know-how required for modern drive development and production mean that as of 2007, over 98% of the world's HDDs are manufactured by just a handful of large firms: Seagate Technology (which now owns Maxtor), Western Digital, Samsung Electronics, and Hitachi Ltd. (which owns the former disk manufacturing division of International Business Machines). Fujitsu continues to make mobile- and server-class disks but exited the desktop-class market in 2001. Toshiba is a major manufacturer of 2.5-inch and 1.8-inch notebook disks. ExcelStor is a small HDD manufacturer.

Dozens of former HDD manufacturers have gone out of business, merged, or closed their HDD divisions; as capacities and demand for products increased, profits became hard to find, and the market underwent significant consolidation in the late 1980s and late 1990s. The first notable casualty of the business in the PC era was Computer Memories Inc. or CMI; after an incident with faulty 20 MB AT disks in 1985,Apparently the CMI disks suffered from a higher soft error rate than IBM's other suppliers (Seagate and MiniScribe) but the bugs in Microsoft's DOS Operating system may have turned these recoverable errors into hard failures. At some point, possibly MS-DOS 3.0, soft errors were reported as disk hard errors and a subsequent Microsoft patch turned soft errors into corrupted memory with unpredictable results ("crashes"). MS-DOS 3.3 apparently resolved this series of problems but by that time it was too late for CMI. See also, "IBM and CMI in Joint Effort to Rehab AT Hard-Disk Rejects," PC Week, v.2 n.11, p.1, March 19, 1985 CMI's reputation never recovered, and they exited the HDD business in 1987. Another notable failure was MiniScribe, who went bankrupt in 1990 after it was found that they had engaged in accounting fraud and inflated sales numbers for several years. Many other smaller companies (like Kalok, Microscience International Corporation, LaPine, Areal, Priam and PrairieTek) also did not survive the shakeout, and had disappeared by 1993; Micropolis Corporation was able to hold on until 1997, and JT Storage, a relative latecomer to the scene, lasted only a few years and was gone by 1999, after attempting to manufacture HDDs in India. Their claim to fame was creating a new 3" form factor drive for use in laptops. Quantum and Integral also invested in the 3" form factor; but eventually gave up as this form factor failed to catch on. Rodime plc was also an important manufacturer during the 1980s, but stopped making disks in the early 1990s amid the shakeout and now concentrates on technology licensing; they hold a number of patents related to 3.5-inch form factor HDDs.



History

For many years, HDDs were large, cumbersome devices, more suited to use in the protected environment of a data center or large office than in a harsh industrial environment (due to their delicacy), or small office or home (due to their size and power consumption). Before the early 1980s, most HDDs had 8-inch (20 cm) or 14-inch (35 cm) platters, required an equipment rack or a large amount of floor space (especially the large removable-media disks, which were often referred to as "washing machines"), and in many cases needed high-current or even three-phase power hookups due to the large motors they used. Because of this, HDDs were not commonly used with microcomputers until after 1980, when Seagate Technology introduced the ST-506, the first 5.25-inch HDD, with a capacity of 5 megabytes. In fact, in its factory configuration, the original IBM PC (IBM 5150) was not equipped with a hard disk drive.

Most microcomputer HDDs in the early 1980s were not sold under their manufacturer's names, but by Original equipment manufacturers as part of larger peripherals (such as the Corvus Disk System and the Apple ProFile). The IBM PC/XT had an internal HDD, however, and this started a trend toward buying "bare" disks (often by mail order) and installing them directly into a system. Hard disk drive makers started marketing to end users as well as OEMs, and by the mid-1990s, HDDs had become available on retail store shelves.

While internal disks became the system of choice on PCs, external HDDs remained popular for much longer on the Apple Macintosh and other platforms. The first Apple Macintosh built between 1984 and 1986 had a closed architecture that did not support an external or internal hard drive. In 1986, Apple added a SCSI port on the back, making external expansion easy. External SCSI drives were also popular with older microcomputers such as the Apple II series, and were also used extensively in server (computing)s, a usage which is still popular today. The appearance in the late 1990s of high-speed external interfaces such as Universal Serial Bus and FireWire has made external disk systems popular among PC users once again, especially for laptop users, users that install Linux in the additional external unit and users who move large amounts of data between two or more areas. Most HDD makers now make their disks available in external cases.

See also

Notes and references External links



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