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Core of each disk are the data disks (or "plates", Eng. "Platters").
On them are the actual data stored, represented by a magnetic state. For this to happen we need a magnetic material: The discs, which themselves are made of aluminum or plastic, are provided with a very thin (~ 1 micron) or cobalt oxide layer. In this magnetizable layer is now using a write head writes the data.
A typical hard disk usually contains several such data discs, which are firmly mounted on a rotatable axis - called spindle. The rigid assembly is important to ensure a smooth, vibration-free operation permit, but the subtle mechanism may be disturbed by small disturbances or rendered unusable. The spindle rotates with the data disks, can be accessed on the data slices with a read-write head. To generate the rotation that is an engine is necessary.