Magnetic hard disks: spinning platters and flying heads

Inside a hard disk drive (HDD), metal or glass discs called platters spin thousands of times a minute. A head on an arm swings across them, like the needle on a record player.

Except this needle never touches the surface. It flies a few nanometres above it, on a cushion of air.

• Platters: both surfaces are coated in a magnetic material. They spin together on a spindle at a constant speed (e.g. 7200 revolutions per minute).
• Each surface is divided into concentric rings called tracks, and each track into sectors (fixed-size blocks of data).
• One read/write head per surface, all fixed to an actuator arm that moves them across the platters.
• Each bit is a tiny area of the coating magnetised in one of two directions: one direction for 1, the other for 0.

"Describe how data is read from (or written to) a hard disk." Usually 3 to 5 marks.

The marks are for finding the data (track, then sector) and then how the head reads or writes the magnetism.

1. The platters spin at a constant high speed.
2. The disk controller works out which track and sector hold the data.
3. The actuator arm moves the read/write head to the correct track.
4. The head waits for the correct sector to spin round underneath it.
5. As the sector passes, the head detects the direction of magnetisation of each tiny area and turns it into an electrical signal: 1s and 0s.
6. The data is sent to the computer.

Step 4 is the forgotten one. Moving the arm only gets you to the right ring; the spin does the rest.