Hardware Reference
In-Depth Information
Of course, as mentioned earlier, today's ATA drives have head, track, and sector-per-track
parameters that are translated from the actual physical numbers. Therefore, you usually
can't tell from the published numbers exactly how many heads or platters are contained
within a drive.
Automatic Head Parking
When you power off an HDD using the CSS design, the spring tension in each head arm
pulls the heads into contact with the platters. The drive is designed to sustain thousands
of takeoffs and landings, but it is wise to ensure that the landing occurs at a spot on
the platter that contains no data. Older drives from the 1980s and early 1990s required
manual head parking; you had to run a program that positioned the drive heads to a land-
ing zone—usually the innermost cylinder—before turning off the system. Modern drives
automatically park the heads, so park programs are no longer necessary.
Some amount of abrasion occurs during the landing and takeoff process, removing just
a “micro puff” from the magnetic medium, but if the drive is jarred during the landing
or takeoff process, real damage can occur. Newer drives that use load/unload designs in-
corporate a ramp positioned outside the outer surface of the platters to prevent contact
between the heads and platters, even if the drive is powered off. Load/unload drives auto-
matically park the heads on the ramp when the drive is powered off.
One benefit of using a voice coil actuator is automatic head parking. In a drive that has a
voice coil actuator, the heads are positioned and held by magnetic force. When the power
to the drive is removed, the magnetic field that holds the heads stationary over a par-
ticular cylinder dissipates, enabling the head rack to skitter across the drive surface and
potentially cause damage. In the voice coil design, the head rack is attached to a weak
spring at one end and a head stop at the other end. When the system is powered on, the
spring is overcome by the magnetic force of the positioner. When the drive is powered
off, however, the spring gently drags the head rack to a park-and-lock position before
the drive slows down and the heads land. On some drives, you could actually hear the
“ting...ting...ting...ting” sound as the heads literally bounce-parked themselves, driven by
this spring.
On a drive with a voice coil actuator, you activate the parking mechanism by turning off
the computer; you do not need to run a program to park or retract the heads, as was ne-
cessary with early hard disk designs. In the event of a power outage, the heads park them-
selves automatically. (The drives unpark automatically when the system is powered on.)
Air Filters
Nearly all HDDs have two air filters. One is called the recirculating filter, and the other
is called either a barometric or breather filter. These filters are permanently sealed inside
