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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.
The data stored on the disk is in steady concentric circles, called tracks. These tracks are divided into clusters which in turn consist of several sectors.
Sectors are usually 512 bytes in size. Although of course the magnetic states of zeros and ones, ie bits representing, for the reading head of the sector, the basic unit, because it will always read entire sectors rather than individual bits.
From the technology of hard drives raises another group: those of the cylinder. The data discs are arranged in parallel so that traces are the same diameter on each other. Anticipation was gripped by the fact that read and write heads are always available as a set and does not move individually, so they are necessarily above the other. From this fact results in a meaningful collection of stacked traces to form cylinders.
In fact, the associated data to be stored so that cylinders are filled first. Also the defrag process attempts to achieve this.