Hardware Reference
In-Depth Information
For example, the Toshiba MK-538FB 1.2GB drive that I used to have in one of my systems had 8
platters, but only 15 read/write heads. That drive uses a dedicated servo positioning system, and the
16th head is the servo head. The advantage of the dedicated servo concept is that the servo
information is continuously available to the drive, making the head positioning process faster and
more precise.
The drawback to a dedicated servo is that dedicating an entire platter surface for servo information is
wasteful. Virtually all drives today use a variation on the embedded servo technique instead. Some
drives combined a dedicated servo with an embedded servo, but this type of hybrid design is rare.
Regardless of whether the servo mechanism is dedicated or embedded, it is far more accurate than the
stepper motor mechanisms of the past.
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 landing 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 incorporate 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 automatically 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 particular 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 necessary with early hard disk
designs. In the event of a power outage, the heads park themselves 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 the drive and are
