Hardware Reference
In-Depth Information
found in PCs today spin at 7,200 rpm, with high-performance models spinning at 10,000 rpm,
although many less-expensive drives still spin at 5,400 rpm. Some of the small 2 1/2-inch notebook
drives run at only 4,200 rpm to conserve power, and 15,000 rpm drives are usually found only in
high-performance workstations or servers, where their higher prices, heat generation, and noise can
be more easily dealt with. High rotational speeds combined with a fast head-positioning mechanism
and more sectors per track are what make one hard disk faster overall than another.
The heads in most HDDs do not (and should not!) touch the platters during normal operation.
However, on most drives, the heads do rest on the platters when the drive is powered off. In most
drives, when the drive is powered off, the heads move to the innermost cylinder, where they land on
the platter surface. This is referred to as
contact start stop
(CSS) design. When the drive is powered
on, the heads slide on the platter surface as they spin up, until a thin cushion of air builds up between
the heads and platter surface, causing the heads to lift off and remain suspended a short distance
above or below the platter. If the air cushion is disturbed by a particle of dust or a shock, the head
can come into contact with the platter while it is spinning at full speed. When contact with the
spinning platters is forceful enough to do damage, the event is called a
head crash
. The result of a
head crash can be anything from a few lost bytes of data to a completely ruined drive. Most drives
have special lubricants on the platters and hardened surfaces that can withstand the daily “takeoffs
and landings” as well as more severe abuse.
Many newer drives do not use CSS design and instead use a load/unload mechanism that does not
allow the heads to contact the platters, even when the drive is powered off. First used in the 2 1/2-
inch form factor notebook or laptop drives where resistance to mechanical shock is more important,
load/unload mechanisms use a ramp positioned just off the outer part of the platter surface, whereas
some newer designs position the ramp near the spindle. When the drive is powered off or in a power-
saving mode, the heads ride up on the ramp. When the drive is powered on, the platters are allowed
to come up to full speed before the heads are released down the ramp, allowing the airflow (air
bearing) to prevent head/platter contact.
Because the platter assemblies are sealed and nonremovable, the track densities on the disk can be
high. Hard drives today have up to 270,000 or more tracks per inch (TPI) recorded on the media.
Head disk assemblies (HDAs), which contain the platters, are assembled and sealed in clean rooms
under absolutely sanitary conditions. Because few companies repair HDAs, repair or replacement of
the parts inside a sealed HDA can be expensive. Every hard disk ever made eventually fails. The
only questions are when the failure will occur and whether your data is backed up.
Caution
It is strongly recommended that you do not even attempt to open an HDD's HDA unless you
have the equipment and expertise to make repairs inside. Most manufacturers deliberately
make the HDA difficult to open to discourage the intrepid do-it-yourselfer. Opening the HDA
voids the drive's warranty.
Note
Although you should never open a disk drive to attempt to repair it, disk drives can operate
without a cover. In some of my PC Hardware and Troubleshooting or Data Recovery seminars,
I have even removed and installed the covers while the drives were operating! Those drives
