The read/write heads of the hard disk float on a cushion of air over the surface of the platters. Each time the power is applied to the drive, the
platters spin up to speed, and the buildup of air under the heads causes them to lift off
the data surfaces. When the power is cut, the platters spin down, the cushion of air
dissipates, and the heads drop back to the surface of the drive. (Actual contact with the
drive normally occurs in a dedicate landing zone,
to ensure the heads don't contact parts of the disk containing user data.)
Each time the drive starts and stops a small amount of wear occurs to the heads and
also to other components such as the spindle motor. For
this reason, hard drives are given a specification for the minimum number of start/stop
cycles they are designed to handle during their service life. The value for a desktop
drive is typically between 30,000 and 50,000 cycles (and remember that this is not an
average, but a minimum). Notebook drives,
which are more commonly spun up and down a great deal to save battery power, usually have
even higher numbers.
This specification, like MTBF and service life, provides a useful clue about the
quality of the hard disk, but should not be sweated over. In the great-and-never-ending debate over whether to leave hard disks running or spin them down when idle, some look at the fact that start/stop cycles are specified as evidence that stop/start cycles are
"bad" and therefore that drives should always be left running 24/7. As always,
maintain perspective: if you start your hard disk in the morning and stop it at night
every day for three years, that's only about 1,000 cycles. Even if you do it ten times a
day, every day, you're not going to get close to the minimum specification for almost any
quality drive, and for notebooks the problem is less significant because the drives are
generally designed to withstand many more cycles.
IBM drives that use its head load/unload technology are
often given a specification for minimum load/unload cycles instead of start/stop
cycles. This is basically the same concept, except that the numbers are typically much
higher. Some IBM notebook drives are spec'ed for 300,000 load/unload cycles! Even if you
started and stopped the drive 100 times a day it would take you over eight years to get to
that number.
Next: Error Rates
