Inside the 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 - These typically spin at 3,600 or 7,200
rpm when the drive is operating. These platters are
manufactured to amazing tolerances and are mirror-smooth

* The arm - This holds the read/write heads and is
controlled by the mechanism in the upper-left corner. The arm
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!

Inside: Platters and Heads

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.

Storing the Data

Data is stored on the surface of a platter in sectors and
tracks. Tracks are concentric circles, and sectors are
pie-shaped wedges on a track, like this:

A typical track is shown in yellow; a typical sector is shown
in blue. A sector contains a fixed number of bytes -- for
example, 256 or 512. Either at the drive or the operating
system level, sectors are often grouped together into
clusters.

The process of low-level formatting a drive establishes the
tracks and sectors on the platter. The starting and ending
points of each sector are written onto the platter. This
process prepares the drive to hold blocks of bytes.
High-level formatting then writes the file-storage structures,
like the file-allocation table, into the sectors. This
process prepares the drive to hold files.