I took apart an old hard drive. The silver disc that is reflecting my camera is called the platter. This is made of a non-magnetic material like aluminium or ceramic with a thin magnetisable layer on the surface.
The actuator arm has a very tiny electromagnetic coil on the pointed end. It is called the read-write head. If current runs through this, it acts as a magnet and magnetises sections of the platter below. When reading data, the magnetised sections moving below it cause electromagnetic induction to take place. An emf is induced which causes a current to flow in the read-write head and the computer can pick up the data.
There are strong permanent magnets in the corner with what is called a voice coil between them. When current flows in the voice coil, it attracts or repels from the permanent magnets and can thus move.
The magnetic circuitry must be very sensitive and will deserve testing next time one comes my way.
