Inside a Hard Disk

The best way to understand how a hard disk works is to take a look inside.

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 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, actually!)
• The arm that holds the read/write heads. This arm 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!

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 moves the arm.