The capacity of hard disk drives is frequently stated by manufacturers in megabytes (1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) or terabytes (1 TB = 1,000,000,000,000 bytes). This numbering convention, where prefixes like kilo- and mega- denote powers of 1000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used in the memory industry (i.e., RAM and ROM) and for CDs where prefixes like kilo- and mega- denote capacity in powers of 1024.
When the unit prefixes like kilo- denote powers of 1024 in the reporting of computer capacities, the 1024n progression (for n = 1, 2, …) is as follows:
* kilo = 210 = 10241 = 1024,
* mega = 220 = 10242 = 1,048,576,
* giga = 230 = 10243 = 1,073,741,824,
and so forth.
The HDD industry practice of using prefixes assigned powers of 1000 to describe HDD capacity (storage) dates back to the earliest days of computing
The Memory industry practice of using the same prefixes but assigned to powers of 1024 to describe memory capacity also dates back to the earliest days of computing.
There is really no reason for this difference besides it just being convention to use powers of 1024 in reporting memory size. The computer itself does not internally represent the HDD (or memory) capacity as being in powers of 1024. Until the 1980s there was little confusion because the use of these prefixes were generally consistent within articles, product literature and marketing materials while operating systems and utilities reported exact decimal HDD capacity as long strings of decimal digits, without prefixes. For unknown reasons, beginning in the 1980s, operating systems and or utilities began reporting HDD capacity using prefixes denoting powers of 1024. Altering this practice to use conventional powers of 1000 could have been done at any time, including from the beginning; however, for some reason it just stuck this way for most of the computing industry.
In the case of “mega-,” there is a nearly 5% difference between its decimal definition used by the HDD industry and the powers-of-two definition used by the semiconductor industry for memory. The difference is compounded by 2.4% with each incrementally larger prefix (gigabyte, terabyte, et cetera). This discrepancy between the two conventions for reporting capacity has led to confusion and litigation.
Different operating systems report HDD capacity in different ways. Most operating systems and associated utilities including Microsoft's Windows use powers of 1024 prefixes to report HDD capacity. In such systems, an HDD specified by its manufacture as 1 TB would be reported as 931 GB leading to confusion over actual HDD capacity. Beginning August 2009 current versions of Apple's MacOS X operating system (version 10.6 and later) report HDD capacity using powers of 1000 prefixes and thereby avoids confusion as to HDD capacity by reporting capacity using the same symbols with the same meaning as the HDD industry.
Plaintiffs in two class action suits against HDD manufacturers argued that the use of decimal measurements (i.e., powers of 1000) effectively misled consumers (see Orin Safier v. Western Digital Corporation and Cho v. Seagate Technology (US) Holdings, Inc.).
In December 1998, an international standards organization attempted to address these dual definitions of the conventional prefixes by proposing unique binary prefixes and prefix symbols to denote multiples of 1024, such as “mebibyte (MiB)”, which exclusively denotes 10242 or 1,048,576 bytes. In the over‑12 years that have since elapsed, the proposal has seen little adoption by the computer industry and the conventionally prefixed forms of “byte” continue to denote slightly different values depending on context.
Form factors
Mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines (e.g. HP 7935 and DEC RP06 Disk Drives) or designed so that dimensions enabled placement in a 19" rack (e.g. Diablo Model 31). In 1962, IBM introduced its model 1311 disk, which used 14 inch (nominal size) platters. This became a standard size for mainframe and minicomputer drives for many years, but such large platters were never used with microprocessor-based systems.
With increasing sales of microcomputers having built in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable, and this led to the evolution of the market towards drives with certain Form factors, initially derived from the sizes of 8-inch, 5.25-inch, and 3.5-inch floppy disk drives. Smaller sizes than 3.5 inches have emerged as popular in the marketplace and/or been decided by various industry groups.
* 8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)
In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD.
* 5.25 inch: 5.75 in × 3.25 in × 8 in (146.1 mm × 82.55 mm × 203 mm)
This smaller form factor, first used in an HDD by Seagate in 1980, was the same size as full-height 5+1⁄4-inch-diameter (130 mm) FDD, 3.25-inches high. This is twice as high as "half height"; i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical 120 mm disks (DVD, CD) use the half height 5¼″ dimension, but it fell out of fashion for HDDs. The Quantum Bigfoot HDD was the last to use it in the late 1990s, with "low-profile" (≈25 mm) and "ultra-low-profile" (≈20 mm) high versions.
* 3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³
This smaller form factor, first used in an HDD by Rodime in 1983,[29] was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today it 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 × 0.275–0.59 in × 3.945 in (69.85 mm × 7–15 mm × 100 mm) = 48.895–104.775 cm3
This smaller form factor was introduced by PrairieTek in 1988;[30] 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. It is also used in the Playstation 3 and Xbox 360[citation needed] video game consoles. Today, the dominant height of this form factor is 9.5 mm for laptop drives (usually having two platters inside), but higher capacity drives have a height of 12.5 mm (usually having three platters). Enterprise-class drives can have a height up to 15 mm. Seagate has released a wafer-thin 7mm drive aimed at entry level laptops and high end netbooks in December 2009.
* 1.8 inch: 54 mm × 8 mm × 71 mm = 30.672 cm³
This form factor, originally introduced by Integral Peripherals in 1993, has evolved into the ATA-7 LIF with dimensions as stated. It was increasingly used in digital audio players and subnotebooks, but is rarely used today. An original variant exists for 2–5GB sized HDDs that fit directly into a PC card expansion slot. These became popular for their use in iPods and other HDD based MP3 players.
* 1 inch: 42.8 mm × 5 mm × 36.4 mm
This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.
* 0.85 inch: 24 mm × 5 mm × 32 mm
Toshiba announced this form factor in January 2004[36] 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 [dead link] and holds the Guinness World Record for the smallest hard disk drive.
3.5-inch and 2.5-inch hard disks currently dominate the market.
By 2009 all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch form factors due to falling prices of flash memory, which is slightly more stable and resistant to damage from impact and/or dropping.
The inch-based nickname of all these form factors usually do not indicate any actual product dimension (which are specified in millimeters for more recent form factors), but just roughly indicate a size relative to disk diameters, in the interest of historic continuity.
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