When the platters are not spinning, the heads rest on the surface of the disk. When the platters spin
up, the heads rub along the surface of the platters until sufficient speed is gained for them to "lift off"
and float on their cushion of air. When the drive is spun down, the process is repeated in reverse. In
each case, for a period of time the heads make contact with the surface of the disk--while in motion,
While the platters and heads are designed with the knowledge in mind that this contact will occur, it
still makes sense to avoid having this happen over an area of disk where there is data! For this reason,
most disks set aside a special track that is designated to be where the heads will be placed for takeoffs
and landings. Appropriately, this area is called the landing zone, and no data is placed there. The process
of moving the heads to this designated area is called head parking.
Most early hard drives that used stepper motors did not automatically park the heads of the drive. As a
safety precaution, small utilities were written that the user would run before shutting down the PC. The
utility would instruct the disk to move the heads to the landing zone, and then the PC could be shut off
safely. A parameter in the BIOS setup for the hard disk told the system which track was the landing
zone for the particular model of hard disk. Usually, it was the next consecutive-numbered track above
the largest-numbered one actually used for data.
Modern voice-coil actuated hard disk drives are all auto-parking. On some disks, a weak spring is
attached to the head assembly that tries to pull the heads to the landing zone. When power is applied
the actuator is able to overpower the spring and position the heads normally. When the power is shut
off, the electromagnetic force from the voice coil abates, and the spring yanks the heads to the landing
zone before the platters can spin down; this can sometimes be heard on older drives as an audible clunk
when you turn the power off. Other disks use a different mechanical or electronic scheme to achieve
the same goal. Some even make use of the rotational energy remaining in the spindle motor to move
the heads off the data surface when the power is cut off! This means that modern hard disks will
automatically park their heads--even in the event of a power failure--and no utilities are required. The
BIOS landing zone parameter for modern drives is ignored.
Some people still think that it is necessary to manually park the heads of modern hard disks, but this
is not true. I sometimes think of head parking utilities as the disk drive's equivalent of a screen saver.
In both cases, the software was invented as a preventative measure, and one that made sense for use
with the technology that prevailed at the time it was thought up. And in both cases, the technology has
evolved to the point where utility is no longer necessary, yet many people still think it is.
IBM has developed an alternative to conventional head parking that I think is really a great idea. Instead
of letting the heads fall down to the surface of the disk when the disk's motor is stopped, the heads are
lifted completely off the surface of the disk while the drive is still spinning, using a special ramp. Only
then are the disks allowed to spin down. When the power is reapplied to the spindle motor, the process is
reversed: the disks spin up, and once they are going fast enough to let the heads fly without contacting
the disk surface, the heads are moved off the "ramp" and back onto the surface of the platters. IBM calls
this load/unload technology. In theory it should improve the reliability of the hard disk as a whole, by
eliminating most contact between the heads and platters entirely. I am unaware of any other drive
manufacturers using it at this time. You can read more about it here.
Another feature related to reducing damage to the hard disks caused by wear from the heads is wear
leveling, which moves the heads over the surface of the drive to avoid "wearing out" one section of the
drive. It is discussed in this quality and reliability section.