Hardware Reference
In-Depth Information
Larger HDDs that have 8-inch, 14-inch, or even larger platters are available, but these
drives are not used with PC systems. Currently, the 3 1/2-inch drives are the most popular
fordesktopandsomeportablesystems,whereasthe21/2-inchandsmallerdrivesarepop-
ular in portable and notebook systems.
Most HDDs have two or more platters, although some of the smaller drives used in port-
able systems and some entry-level drives for desktop computers have only one. The num-
ber of platters a drive can have is limited by the drive's vertical physical size. The max-
imum number of platters I have seen in any 3 1/2-inch drive is 12; however, most drives
have six or fewer.
Platters were originally made from an aluminum/magnesium alloy, which provides both
strength and light weight. However, manufacturers' desire for higher and higher densities
and smaller drives has led to the use of platters made of glass (or, more technically, a
glass-ceramic composite). Onesuchmaterial, producedbytheDowCorningCorporation,
is called MemCor. MemCor is composed of glass with ceramic implants, enabling it to
resist cracking better than pure glass. Glass platters offer greater rigidity than metal (be-
cause metal can be bent and glass can't) and can therefore be machined to one-half the
thickness of conventional aluminum disks—sometimes less. Glass platters are also much
more thermally stable than aluminum platters, which means they do not expand or con-
tract much with changes in temperature. Virtually all modern drives use glass or glass-
ceramic platters.
Recording Media
No matter which substrate is used, the platters are covered with a thin layer of a magnet-
ically retentive substance, called the
medium
, on which magnetic information is stored.
Three popular types of magnetic media have been used on hard disk platters:
