grinding away

Hard disk drive


HDDs (introduced in 1956 as data storage for an IBM accounting computer) were originally developed for use with general purpose computers. In the 21st century, applications for HDDs have expanded 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 and Nokia. The need for large-scale, reliable storage, independent of a particular device, led to the introduction of embedded systems such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

Capacity and access speed

Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk drive. Consequently, hard disk drives can store much more data than floppy disk drives and can access and transmit it faster. As of January 2008:

  • A typical desktop HDD, might store between 120 and 1000 GB of data (based on US market data), rotate at 5,400 to 10,000 rpm and have a media transfer rate of 1 Gbit/s or higher . (1 GB = 109 B; 1 Gbit/s = 109 bit/s)
  • , the highest capacity HDDs are 1.5  TB.
  • The fastest “enterprise” HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s. and a sustained transfer rate up to 125 MBytes/second. Drives running at 10,000 or 15,000 rpm use smaller platters because of air drag and therefore generally have lower capacity than the highest capacity desktop drives.
  • Mobile, i.e., laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. A typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium. Because of the smaller disks, mobile HDDs generally have lower capacity than the highest capacity desktop drives.

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 digital video recorders and digital audio players. In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based services with extraordinary capacity requirements, such as free-of-charge web search, web archiving and video sharing (Google, Yahoo!, YouTube, etc.).

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. Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

The first 3.5″ HDD marketed as able to store 1 TB was the Hitachi Deskstar 7K1000. It contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space. Hitachi has since been joined by Samsung (Samsung SpinPoint F1, which has 3 × 334 GB platters), Seagate and Western Digital in the 1 TB drive market.

Form factor Width Largest capacity Platters (Max)
5.25″ FH 146 mm 47 GB (1998) 14
5.25″ HH 146 mm 19.3 GB (1998) 4
3.5″ 102 mm 1.5 TB (2008) 5
2.5″ 69.9 mm 500 GB (2008) 3
1.8″ (CE-ATA/ZIF) 54 mm 250 GB (2008) 3
1.3″ 43 mm 40 GB (2007) 1
1″ (CFII/ZIF/IDE-Flex) 42 mm 20 GB (2006) 1
0.85″ 24 mm 8 GB (2004) 1

Capacity measurements

Capacity of a hard disk drive is usually quoted in gigabytes and terabytes. Older HDDs quoted their smaller capacities in megabytes, some of the first drives for PCs being just 5 or 10 MB.



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

Most operating-system tools report capacity using the same abbreviations but actually use binary prefixes. For instance, the prefix mega-, which normally means 106 (1,000,000), in the context of data storage can mean 220 (1,048,576), which is nearly 5% more. Similar usage has been applied to prefixes of greater magnitude. This results in a discrepancy between the disk manufacturer's stated capacity and the apparent capacity of the drive when examined through most operating-system tools. The difference becomes even more noticeable for a gigabyte (7%), and again for a terabyte (9%). For a petabyte there is a 11% difference between the SI (10005) and binary (10245) definitions. For example, Microsoft Windows reports disk capacity both in decimal-based units to 12 or more significant digits and with binary-based units to three 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 Microsoft Windows, Mac OS and some Linux distributions use "gigabyte" for 1,073,741,824 bytes (230 bytes), the operating system reports capacity of the disk drive as (only) 28.0 GB.

Form factors

The earliest “form factor” hard disk drives inherited their dimensions from floppy-disk drives (FDDs), so that either could be mounted in chassis slots, and thus the HDD form factors became colloquially named after the corresponding FDD types. "Form factor" compatibility continued after the 3½ inch size even though floppy disk drives with new smaller dimensions ceased to be offered.

    In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD. Both "full height" and "half height" (2.313 in) versions were available.
    This smaller form factor, first used in an HDD by Rodime in 1984, was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today has been largely superseded by 1-inch high “slimline” or “low-profile” versions of this form factor which is used by most desktop HDDs.
  • 2.5 inch: 2.75 in × 0.374–0.59 in × 3.945 in (69.85 mm × 9.5–15 mm × 100 mm) = 66.3575cm³-104.775cm³
    This smaller form factor was introduced by PrairieTek in 1988; there is no corresponding FDD. It is widely used today for hard-disk drives in mobile devices (laptops, music players, etc.) and as of 2008 replacing 3.5 inch enterprise-class drives. Today, the dominant height of this form factor is 9.5 mm for laptop drives, but high capacity drives have a height of 12.5 mm. Enterprise-class drives can have a height up to 15 mm.
  • 1.8 inch: 54 mm × 8 mm × 71 mm = 30.672cm³
  • 1 inch: 42.8 mm × 5 mm × 36.4 mm
  • 0.85 inch: 24 mm × 5 mm × 32 mm
    Toshiba announced this form factor in January 2004 for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB (MK8003MTD) version and holds the Guinness World Record for the smallest harddisk drive.

Major manufacturers discontinued the development of new products for the 1-inch (1.3-inch) and 0.85-inch form factors in 2007, due to falling prices of flash memory, although Samsung introduced in 2008 with the SpinPoint A1 another 1.3-inch drive.


Other characteristics

Data transfer rate (as of 2008) 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. In contrast, the first PC drives could manage only around 40 KiB/s.

Seek time currently ranges from just under 5 ms for high-end server drives, to 15 ms for miniature drives, with the most common desktop type typically being around 9 ms. There has not been any significant improvement in this speed for some years. Some early PC drives used a worm-gear to move the heads, and as a result had access times as slow as 80–120 ms, but this was quickly improved by voice-coil type actuation in the late 1980s, seeing access times reduce to around 20 ms.


Audible noise (measured in dBA) is significant for certain applications, such as PVRs digital audio recording and quiet computers. Low noise disks typically use fluid bearings, slower rotational speeds (usually 5,400 rpm) and reduce the seek speed under load (AAM) to reduce audible clicks and crunching sounds. Drives in smaller form factors (e.g. 2.5 inch) are often quieter than larger drives.

Shock resistance is especially important for mobile devices. Some laptops now include a motion sensor that parks the disk heads if the machine is dropped, hopefully before impact, to offer the greatest possible chance of survival in such an event.

Access and interfaces

Hard disk drives are accessed over one of a number of bus types, including parallel ATA (PATA, 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 buses that they cannot communicate with natively, such as IEEE 1394 and USB.

Back in the days of the ST-506 interface, the data encoding 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 RLL (or just "RLL") encoding increased the transfer rate by 50%, to 7.5 megabits per second; this 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.)



SCSI originally had just one speed, 5 MHz (for a maximum data rate of five 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).



Disk interface families used in personal computers

Notable families of disk interfaces include:

  • Historical bit serial interfaces — connected to a hard disk drive controller with three cables, one for data, one for control and one for power. The HDD controller provided significant functions such as serial to parallel conversion, data separation and track formatting, and required matching to the drive in order to assure reliability.
    • ST412 was available in either MFM or RLL (Run Length Limited) variants.
    • Enhanced Small Disk Interface (ESDI) was an interface developed by Maxtor to allow faster communication between the PC and the disk than MFM or RLL.
  • Modern bit serial interfaces — connect to a host bus adapter (today typically integrated into the "south bridge") with two cables, one for data/control and one for power.
    • Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. The same differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.
    • Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.
  • Word serial interfaces — connect to a host bus adapter (today typically integrated into the "south bridge") with two cables, one for data/control and one for power. The earliest versions of these interfaces typically had a 16 bit parallel data transfer to/from the drive and there are 8 and 32 bit variants. Modern versions have serial data transfer. The word nature of data transfer makes the design of a host bus adapter significantly simpler than that of the precursor HDD controller.
    • Integrated Drive Electronics (IDE), later renamed to ATA, and then later to PATA ("parallel ATA", to distinguish it from the new Serial ATA). The original name reflected the innovative integration of HDD controller with HDD itself, which was not found in earlier disks. Moving the HDD controller from the interface card to the disk drive helped to standardize interfaces, including reducing the cost and complexity. The 40 pin IDE/ATA connection of PATA transfers 16 bits of data at a time on the data cable. The data cable was originally 40 conductor, but later higher speed requirements for data transfer to and from the hard drive led to an "ultra DMA" mode, known as UDMA. Progressively faster versions of this standard ultimately added the requirement for an 80 conductor variant of the same cable; the other conductors provided the grounding necessary for enhanced high-speed signal quality by the reduction of cross talk. The interface for 80 conductor only has 39 pins, the missing pin acting as a key to prevent incorrect insertion of the connector to an incompatible socket, a common cause of disk and controller damage.
    • EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA does not require the CPU/program/operating system to leave other tasks idle while the data transfer occurs.

Acronym or abbreviation Meaning Description
SASI Shugart Associates System Interface Historical predecessor to SCSI.
SCSI Small Computer System Interface Bus oriented that handles concurrent operations.
SAS Serial Attached SCSI Improvement of SCSI, uses serial communication instead of parallel.
ST-506 Historical Seagate interface.
ST-412 Historical Seagate interface (minor improvement over ST-506).
ESDI Enhanced Small Disk Interface Historical; backwards compatible with ST-412/506, but faster and more integrated.
ATA Advanced Technology Attachment Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations.
SATA Serial ATA Modification of ATA, uses serial communication instead of parallel.



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 3,000 m (10,000 feet). Note that modern commercial aircraft have a pressurized cabin, whose pressure altitude does not normally exceed 2,600 m(8,500 feet) - thus, ordinary hard drives can safely be used in flight. Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disk drives — they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating drive 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.


The hard drive'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 and load/unload technology

Most HDDs prevent power interruptions from shutting the drive down with its heads landing in the data zone by either moving the heads to a landing zone or unloading (i.e., load/unload) the heads.



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 (2007), predominantly in desktop and enterprise (3.5 inch) drives. In general, CSS technology can be prone to increased stiction (the tendency for the heads to stick to the platter surface), e.g. as a consequence of increased humidity. Excessive stiction can cause physical damage to the platter and slider or spindle motor.


All HDDs today still use one of these two technologies. Each has a list of advantages and drawbacks in terms of loss of storage area on the disk, relative difficulty of mechanical tolerance control, non-operating shock robustness, cost of implementation, etc.


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 attempts 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 will lead to the loss of data. While it may sometimes be possible to recover lost information, it is normally an extremely costly procedure, and it is not possible to guarantee success. A 2007 study published by Google suggested very little correlation between failure rates and either high temperature or activity level; however, the correlation between manufacturer/model and failure rate was relatively strong. Google did not publish the manufacturer's names along with their respective failure rates. 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.

A common misconception is that a colder hard drive will last longer than a hotter hard drive. The Google study seems to imply the reverse -- "lower temperatures are associated with higher failure rates". Hard drives with S.M.A.R.T.-reported average temperatures below 27 °C had failure rates worse than hard drives with the highest reported average temperature of 50 °C, failure rates at least twice as high as the optimum S.M.A.R.T.-reported temperature range of 36 °C to 47 °C.


The mean time between failures (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 1.5 million hours. However, independent research indicates that 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 before it enters 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.


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



See also List of defunct hard disk manufacturers

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 (which now owns Maxtor), Western Digital, Samsung, and Hitachi (which owns the former disk manufacturing division of IBM). Fujitsu continues to make mobile- and server-class disks but exited the desktop-class market in 2001, and is reportedly selling the rest to Western Digital Toshiba is a major manufacturer of 2.5-inch and 1.8-inch notebook disks. ExcelStor is a small HDD manufacturer.


  • 1988: Tandon Corporation sold its disk manufacturing division to Western Digital (WDC), which was then a well-known controller designer.
  • 1989: Seagate Technology bought Control Data's high-end disk business, as part of CDC's exit from hardware manufacturing.
  • 1990: Maxtor buys MiniScribe out of bankruptcy, making it the core of its low-end disk division.
  • 1994: Quantum bought DEC's storage division, giving it a high-end disk range to go with its more consumer-oriented ProDrive range, as well as the DLT tape drive range.
  • 1995: Conner Peripherals, which was founded by one of Seagate Technology's co-founders along with personnel from MiniScribe, announces a merger with Seagate, which was completed in early 1996.
  • 1996: JTS merges with Atari, allowing JTS to bring its disk range into production. Atari was sold to Hasbro in 1998, while JTS itself went bankrupt in 1999.
  • 2000: Quantum sells its disk division to Maxtor to concentrate on tape drives and backup equipment.
  • 2003: Following the controversy over mass failures of its Deskstar 75GXP range, HDD pioneer IBM sold the majority of its disk division to Hitachi, who renamed it Hitachi Global Storage Technologies (HGST).
  • 2003: Western Digital purchased Read-Rite Corp, which makes recording heads used on disk drive platters, for $95.4 million in cash.
  • December 21, 2005: Seagate and Maxtor announced an agreement under which Seagate would acquire Maxtor in an all stock transaction valued at $1.9 billion. The acquisition was approved by the appropriate regulatory bodies, and closed on May 19, 2006.
  • 2007
    • July: Western Digital (WDC) acquires Komag U.S.A, a thin-film media manufacturer, for USD 1 Billion.

See also

Notes and References

External links

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