An opened harddisk looks like this:
A hard disk consists of one or several rotary magnetic disks, an impulse, movable heads, a tax electronics with an interface to the connection with the computer.
It is interesting to work out that at a radius of 40 mm of the magnetic disks and at a rotating speed of 7200 revs, the heads race at a speed of 108 km/h and at a distance of approx. 0.2 µ m above the magnet surface. For comparison: a fingerprint has the height of 4 µ m.
Any resemblance to a record player is only formal. The data are not stored on the disk successively like songs on a record. In order to reach bigger capacities and short access times, the data are distributed in a certain algorithm on the magnetic surfaces.
The write/ read head exists of two heads: Writing head and reading head. Due to the rotation of the write/ read unity these both heads are on different tracks. The synchronisation of both heads is guaranteed by the hard disk programme (Microcode). The parametres of this programme are summarised in the production and are individual for every single hard disk.