Hard Drives- How They Work
All hard drives share the same basic structure, varying only in
how each part is used and the quality of the parts themselves. The
platters, spindle motor, heads, and head actuator are inside the
drive, all sealed from the outside environment. This chamber is
often called the head disk assembly (HDA). The HDA is rarely opened,
except by professionals. On the outside are the logic board, bezel,
and mounting equipment. Each of these components are described below:
The platters are the disks inside the drive. Platters can vary
in size. Often the size of the drive, 5.25" or 3.5", is based on
the physical size of the platters. Most drives have several platters.
They are usually made of an aluminum alloy so that they are light.
The newest and largest drives make use of a new technology of glass/ceramic
platters. Basically, this is glass with enough ceramic within to
resist cracking. This glass technology is taking over aluminum in
the hard drive industry. Many popular manufacturers already use
it, including Maxtor, Toshiba, and Seagate. Glass platters can be
made much thinner than aluminum ones, they can better resist the
heat produced during operation and they are also better able to
withstand the extreme centrifugal forces during spinning on the
spindle. The platters are mounted onto a spindle in the interior
of the HDA.
Alone, platters are not capable of recording data. Each one is
coated with a film of some magnetically sensitive substance. The
oxide media is one of the older ways of doing this. With this, a
mixture of compound syrup is poured onto the platter, and then spun
to evenly distribute the film over the entire platter. This substance
has iron oxide as a main ingredient, explaining why many platters
you may see will be brownish-orange. Using an oxide coating developed
limits over time as capacities increased. What's more, the oxide
medium could not survive a head crash at all, and usually this required
drive replacement. The more modernly used media consists of a thin
film of a cobalt alloy, which is placed on the platter through electroplating,
much like chrome. This media is then coated with a thin layer of
protective substance to allow some measure of protection against
head crash. Overall, this new medium is much flatter at the microscopic
level, allowing the heads to run closer to the platter (more on
this in a bit).
The read/write heads do just that; they read and write to the platters.
There is usually one head per platter side, and each head is attached
to a single actuator shaft so that all the heads move in unison.
Each head is spring loaded to force it into the platter it reads.
When off, each head rests on the platter surface. When the drive
is running, the spinning of the platters causes air pressure that
lifts the heads ever-so-slightly off the platter surface. The distance
between the head and platter is very small...so small that the HDA
must be assembled in a clean room because one dust particle can
throw the whole thing off. This sensitivity and accuracy is what
causes only bigger companies to be able to repair hard drives simply
because of the expense of a clean room. A slider is attached to
each head. This mechanism actually glides over the platter and holds
the head at the correct distance to do its job. You can see a full
head assembly to the right. You can see the sliders on the end of
each head.
Airflow inside of the drive plays an important part in overall
operation. In fact, air is what holds the heads off of the platters
while the drive is on. The rotation of the platters creates airflow.
The heads fly like an airplane on this cushion of air. The air is
dragged along with the platters by friction. The high-pressure air
between the heads and platters form what is called an air bearing.
The concept is similar to a hockey puck gliding over an air hockey
table.
The read/write heads are bonded to what are called head arms. The
head arms hold the heads over or under the platters. The head arm
and head combination is called the head-gimbal assembly (HGA). The
individual HGA's are mounted together onto an actuator shaft, which
serves as a center of rotation. The head actuator is situated on
the other end of the HGA and is responsible for the movement of
the heads around the platters.
Head actuators come in three types:
 The
stepper motor design is actually an electric motor that
moves from one stop position to another, governed by click stop
positions. They cannot stop between stop positions. The motor
is small and is located outside the HDA, so it is visible from
the outside. The stepper motor design is inferior. It suffers
from slow access rate and is very sensitive to temperature. It
is also sensitive to physical orientation and can't automatically
park the heads in a safe zone. Besides, the actuator operates
blindly from the track positions, governed only by the stop positions.
Over time, the drive becomes misaligned, requiring occasional
re-formats to realign the sector data with the heads.
- The servo motor actuator is another type of head actuator
motor. Unlike the stepper design, the heads get feedback as to
position, assuring the proper tracks are read. The guidance system
used by the heads is called a servo. Its job is to position the
head over the correct cylinder. It does this through the use of
Grey code. Grey code is a special binary number system in which
any two adjacent numbers provide info to the servo as to their
position on the drive. Also, the heads are free to move wherever
needed...no steps. Basically, when the drive needs to retrieve
certain data, the servo motor moves the heads out to the appropriate
position on the disk and then waits for the corrects bit of data
to spin over to it. The time it takes for all this to happen is
called latency, and is a key measure of the speed of the drive.
- The voice coil is the latest type of device for controlling
the heads and is used on all modern hard disks. The voice coil
operates similarly to a speaker. There is a magnet (or magnets)
that is surrounded by a spring-loaded coil of wire, which is connected
to the HGA. As a current is applied to the coil, it interacts
with the magnet and swings the assembly. The resulting movement
of the heads is from the center of the platters to the outer edges.
When the hard drive is powered down, the springs or actuator coil
(depending on the type of actuator) attached to the heads pull the
head into the platter. This is called a landing. Every drive is
designed to handle thousands of takeoffs and landings, but since
the head actually hits the platter, its best to have this happen
on a section of platter where there is no data. In a voice coil
design, the actuator coil springs the heads into a landing zone
and lock position before the drive even stops spinning. The landing
zone typically lies on the innermost cylinder or the outermost cylinder.
This assures that the heads are not just let go of and left to drag
along the platter until the platter stops, a problem common to the
stepper motor design. When powered on, the drive automatically unparks
itself and the coil is overcome by the magnetic force.
The spindle motor is responsible for spinning the platters. These
devices must be precisely controlled and quiet. They are set to
spin the platters at a set rate, ranging from 5400 RPM to 10000
RPM. The motor is attached to a feedback loop to make sure it spins
at exactly the speed it is supposed to. The speed is not adjustable
during operation. Some spindle motors are on the bottom of the drive,
below the HDA, while the more modern ones are built into the hub
of rotation of the platters, thereby taking up no vertical space
and allowing more platters. Attached to the spindle motor is a ground
strap, which helps rid the drive of the static charges, created
by the rotating the platters through the air in the drive's interior.
In many drives, this can be accessed by removing the logic board.
After a while, this strap can become worn and produce noise, like
a high-pitched squeal. One can usually lubricate the strap and stop
the noise, but this entails some minor disassembling of the drive.
The logic board is the board of chips underneath the drive. It
controls the spindle and head actuator and also translates data
to a form usable by the controller and the rest of the system. Some
logic boards have an integrated controller, also. Sometimes, an
apparent disk failure is actually a failure of the logic board.
In such a case, you can replace the logic board and regain access
to the data held up on the drive. This is relatively easy to do,
because the board is simply plugged into the drive and held in by
screws.
Data Storage
I have discussed the structure of a platter
itself, but how exact is data stored on that? First, the heads are
actually small electromagnets. To write information to the platter,
a small electrical current is pulsed from the electronics of the
drive to the head. The direction of the current, and thus the direction
of the magnetic field between the head and the platter, determine
whether or not the data bit is a 1 or a 0 in binary (which is the
only language a PC understands). This magnetic field alters the
direction of the little magnets on that portion of the platter.
The "little magnets" are actually microscopic, the size of a molecule.
The direction of the magnet is retained over time and remains that
way unless rewritten to reflect something different. Thus, the hard
drive will retain this data even after it is powered off.
Capacity
Just take a trip to the computer store and you can see that there
have been major advancements in hard drive technology that lead
to larger capacity drives. Where there were once 500MB drives, we
now have huge drives. Recently, a 36GB IDE drive was released. What
led to this?
Well, the first thought would be: Add more platters, or maybe bigger
ones. Well, yeah, larger platters would do the trick. 5.25" platters
have been used on older drives, and do hold more data. But, manufacturers
don't use these big platters because of the extra stresses the larger
platters put on the motor. These stresses, and the simple fact that
the heads have more disk to cover, make the drives hotter and a
lot bigger. Most drives in use today use 3.5" platters, and 2.5"
or smaller is commonly used for notebook systems. So, the manufacturer
decides it would be better to keep smaller platters, but just add
more of them. This works, but in order to reach such high capacities,
you'd need a lot of platters. The vertical heights of these drives
would just be too much.
Keyword: Areal Density. This is the closeness of data bits on the
hard disk to each other, usually measured in megabytes per square
inch (MBSI). There are a few factors that go into determining the
degree of areal density that is possible:
- Size of the magnetic particles in the platter itself
- Size of the read/write heads. Smaller heads allow larger densities.
- Altitude" of the heads over the platter surface.
This density increases performance and allows more data to be packed
onto a platter. The closeness of the bits together means that more data
passes the head at a time, increasing read/write performance. By combining
this with increase platter rotation speeds, latencies will be lower and
speed will be higher.
Information source: PCMECH.COM
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