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3.5" Form Factor
The 3.5" form factor is the standard in the PC world today, and has been for about the last decade. Drives of this size are found almost exclusively now in modern desktop PCs, and even in servers and larger machines. The only major market where other form factors hold sway over 3.5" is that for laptops and other portable devices, where the reduced size of 2.5" and smaller form factors is important.
Like the 5.25" form factor before it, the 3.5" form factor is named not for any dimension of the drives themselves, but rather for the fact that they were designed to fit into the same drive bay as 3.5" floppy disk drives. 3.5" form factor drives traditionally have used 3.74" platters with an overall drive width of 4.0" and depth of about 5.75". In recent years, 3.5" form factor drives with platters smaller than 3.74"--in some cases much smaller--have appeared on the market. Most 10,000 RPM spindle speed drives reduce the size of the platters to 3", and the new 15,000 RPM Seagate drive has platters just 2.5" in diameter. The shrinking media size is done for performance reasons, but the 3.5" form factor is maintained for compatibility (these high-end drives are designed to go into expensive servers, not laptops!) For this reason, it is no longer the case that you can tell the size of a drive's platters by its form factor. See the discussion of platter size for more details.
3.5" form factor drives come in two general profiles: the larger is the so-called half-height drive, which is 1.63" in height. This name is kind of funny, since it is "half" of a height that never existed for 3.5" form factor drives. The name was derived from the fact that these drives are the same height as half-height 5.25" form factor drives, which are half the height of full-height 3.25" high drives in that form factor. Half-height 3.5" form factor drives are still used today, but only in servers and other high-end platforms. The standard for 3.5" is 1" height, which is commonly called slimline or low-profile, but just as commonly given no name at all and assumed as the default. The reason for the smaller size being the standard is that 1" is the height of a standard 3.5" floppy disk drive and 3.5" drive bay. In addition, there are some drives that are reduced in size from the 1" standard, using for example 0.75" height. Here are the standard profiles for 3.5" form factor drives:
It is likely that the 3.5" form factor will continue to be the industry standard in PCs for years to come, due to the enormous installed base of systems that use this size, and no real compelling reason to change the form factor for the typical desktop machine.
Next: 2.5" Form Factor