Inside a Hard Disk
The best way to understand how a hard disk works
is to take a look inside. (Note that OPENING A
HARD DISK RUINS IT, so this is not something
to try at home unless you have a defunct drive.)
Here is a typical hard-disk drive:
It is a sealed aluminum box with controller
electronics attached to one side. The electronics
control the read/write mechanism and the motor
that spins the platters. The electronics also
assemble the magnetic domains on the drive into
bytes (reading) and turn bytes into magnetic
domains (writing). The electronics are all
contained on a small board that detaches from the
rest of the drive:
Underneath the board are the connections for
the motor that spins the platters, as well as a
highly-filtered vent hole that lets internal and
external air pressures equalize:
Removing the cover from the drive reveals an
extremely simple but very precise interior:
In this picture you can see:
- The platters, which typically spin
at 3,600, 5,400, or 7,200 rpm when the drive is
operating. These platters are manufactured to
amazing tolerances and are mirror-smooth (as
you can see in this interesting self-portrait
of the author... no easy way to avoid that!).
- The arm that holds the read/write
heads is controlled by the mechanism in the
upper-left corner, and is able to move the
heads from the hub to the edge of the drive.
The arm and its movement mechanism are
extremely light and fast. The arm on a
typical hard-disk drive can move from hub to
edge and back up to 50 times per second -- it
is an amazing thing to watch!
In order to increase the amount of information
the drive can store, most hard disks have
multiple platters. This drive has three
platters and six read/write heads:
The mechanism that moves the arms on a hard
disk has to be incredibly fast and precise. It can
be constructed using a high-speed linear motor.
Many drives use a "voice coil" approach
-- the same technique used to move the cone of a
speaker on your stereo is used to move the arm.
