Most hard disks are designed to be installed on the inside of the PC, and are produced
in one of a dozen or so standard sizes and shapes. These standards are called hard disk form
factors and refer primarily to its external dimensions. The reason for standardizing
on form factors is compatibility. Without these standards, hard disks would have to be
custom-made to fit different PCs. By agreeing on standards shapes and sizes for hard
disks--as well as standard interfaces of course--it is
possible for any of the thousands of PC makers to purchase units from any hard disk
manufacturer and know that there won't be problems with fit or form during installation.
Over the life of the PC there have only been a few different hard disk form factors.
Since changing a form factor standard requires coordination from the makers of other
components (such as the makers of system cases) there is
resistance in the industry to change the standard sizes unless there is a compelling
reason to do so. (For example, when laptop PCs became popular new, smaller drives were
created to save space and power, important goals in the world of mobile computing.)
Form factors are generally described by a single metric. For example, the most common
form factors today are "3.5-inch" and "2.5-inch". These numbers
generally refer to the width of the drive, but they can be both vague and
misleading (nice, huh? 
) They usually were chosen for historical reasons and in
typically were based on either the platter size of drives that use the form factor, or the
width of drives using that form factor. Obviously a single number cannot represent both,
and in some cases, it represents neither! For example, 3.5" hard disks are generally
4" wide and use 3.74" platters. (The name in this case comes from the fact
that the drives fit in the same space as a 3.5"
floppy disk drive!) Much more about the relationship between form factors and platters
can be found in the discussion of platter size in the media
section. You will also find there a detailed description of the trend towards smaller
platters in modern hard disks.
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The five most popular internal form factors for PC hard
disks.
Clockwise from the left: 5.25", 3.5", 2.5", PC Card and CompactFlash. |
In this section I examine the major form factors that have been used for internal hard
drives in PCs. This includes details on the dimensions of the form factor, especially the
different heights associated with each. (Most form factors are actually a family
of form factors, with different drives varying in the height dimension). In addition to
the standard internal drive form factors, I briefly discuss external drives and also
removable drive trays, which are sort of a "hybrid" of internal and external
designs.
Note: There may be drives
available in form factors other than those listed here; I believe I have them all but as
always, could be wrong.
Next: 5.25" Form Factor
