A 'hard' disk was invented because computers needed a method of storing data even with the power switched off.

Hard disk work by storing data in magnetic form on the surface of a flat disk or 'platter' as they are called.

Modern hard disks have many platters within them.

Each platter spins around at thousands of revolutions per minute which is why you hear a whirring noise when you listen to a working hard disk.

The idea is to spin the platter whilst a 'head' floats just above the surface. The head reads magnetic data as it moves beneath. The head can also write data on to the magnetic surface.

A basic modern hard disk holds perhaps 60 Gigabytes whilst the more expensive ones can hold 250 Gigabytes or more.