The size of hard drives are traditionally given in inches. It is not exact size specifications, but a form factor. Common form factors for the width are 5.25 inches for the height for example, ⅜ inches. The tariff data is most approximate to the diameter of the disk, not the width of the drive housing. Mostly, however, smaller plates are used, to allow for higher speeds.
In the course of technical development sizes were again discontinued in favor of smaller, because they are less vulnerable to shocks in addition to the saving in space and have lower power consumption. Although less space means first, that a USB SPI i2c drive has smaller plates and thus less space is available. However, the rapid development of technology towards higher data densities compensates for this limitation according to experience short term.
The first hard disk drive IBM 350 of 1956 had a size of 24 inches. Mid-1970 models came with a size of 8 inches, which is also quite fast by the much handier and above all, ease of 5.25 inches were replaced hard disk drives. In between there were sizes of 14 inches and 9 inches.
Some hard drives were introduced in 1980 by Seagate, since 1997, this species is no longer found. Some SCSI server drives as well as the low cost USB SPI i2c drive from Quantum BigFoot were the last representatives of this format. The sizes of these drives are based on those of 5.25 inches floppy disk drives: The width of these drives is 5 ¾ inches (146.05 mm), the height (occupy the two slots) for drives with full height, at half-height drives 1 ⅝ inches (41.275 mm).
There were models with even lower overall height: BigFoot models of the series had a height of ¾ inches (19.05 mm) and 1 inches (25.4 mm). The depth of 5.25 inches disks is not fixed, but should not be much above 200 mm.
3.5 inches hard drives were 1987 (IBM PS / 2 Model 50) introduced and are currently (2012) standard in desktop computers. The sizes of these drives are based on those of 3.5 inches floppy disk drives.
The width of these drives is 4 inches (101.6 mm), the height of most 1 inches (25.4mm). Seagate ST1181677 brought with a hard disk with 12 disks and 1.6 inches (40.64mm) Height out. The depth of 3.5 inches hard drives is 5 ¾ inches (146.05 mm).
The 2.5 inches hard drives were originally designed for notebooks, but are now used in servers and specialized equipment (multimedia players, USB hard drives) use. The width is 70 mm, depth 100 mm.