Di{git}ctionary
All hard drives share a basic structure and are composed of the same physical
features. However, not all hard drives perform the same way as the quality of
the parts of the hard drive will affect its performance. Following is a
description of the common features of the hard drive and how each part works in
relation to the others. Hard drives are extremely sensitive equipment and the
internal workings of a hard drive should not be handled by anyone other than an
experienced professional.
The Platters
The platters are the actual disks inside the drive that store the magnetized
data. Traditionally platters are made of a light alluminum alloy and coated with
a magnetizable material such as a ferrite compound that is applied in liquid
form and spun evenly across the platter or thin metal film plating that is
applied to the platter through electroplating, the same way that chrome is
produced. Newer technology uses glass and/or ceramic platters because they can
be made thinner and also because they are more efficient at resisting heat. The
magnetic layer on the platters has tiny domains of magnetization that are
oriented to store information that is transferred through the read/write heads.
Most drives have at least two platters, and the larger the storage capacity of
the drive, the more platters there are. Each platter is magnetized on each side,
so a drive with 2 platters has 4 sides to store data.
The Spindle and Spindle Motor
The platters in a drive are separated by disk spacers and are clamped to a
rotating spindle that turns all the platters in unison. The spindle motor is
built right into the spindle or mounted directly below it and spins the platters
at a constant set rate ranging from 3,600 to 7,200 RPM. The motor is attached to
a feedback loop to ensure that it spins at precisely the speed it is supposed
to.
The Read/Write Heads
The read/write heads read and write data to the
platters. There is typically one head per platter side, and each head is
attached to a single actuator shaft so that all the heads move in unison. When
one head is over a track, all the other heads are at the same location over
their respective surfaces. Typically, only one of the heads is active at a time,
i.e., reading or writing data. When not in use, the heads rest on the stationary
platters, but when in motion the spinning of the platters create air pressure
that lifts the heads off the platters. The space between the platter and the
head is so minute that even one dust particle or a fingerprint could disable the
spin. This necessitates that hard drive assembly be done in a clean
room. When the platters cease spinning the heads come to rest, or park, at a
predetermined position on the heads, called the landing
zone.
The Head Actuator
All the heads are attached to a single head actuator, or actuator arm,
that moves the heads around the platters. Older hard drives used a stepper
motor actuator, which moved the heads based on a motor reacting to stepper
pulses. Each pulse moved the actuator over the platters in predefined steps.
Stepper motor actuators are not used in modern drives because they are prone to
alignment problems and are highly sensitive to heat. Modern hard drives use a voice
coil actuator, which controls the movement of a coil toward or away from a
permanent magnet based on the amount of current flowing through it. This
guidance system is called a servo.
The platters, spindle, spindle motor, head actuator and the read/write heads
are all contained in a chamber called the head disk assembly (HDA). Outside of
the HDA is the logic board that controls the movements of the internal parts and
controls the movement of data into and out of the drive.
For more information, see:
Internal
Hard Drive Parts
Bibiesse, Club - Di{git}ctionary - 1.3.1
