An open hard drive looks something like this.:
A hard disk consists of one or more rotating magnetic disks, a drive, a movable read-write head arm, control electronics with an interface for connection to 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 rpm, the heads race at a speed of 108 km/h and at a distance of approx. 0.2 µm above the magnetic surface. For comparison, a human fingerprint is 4 µm height.
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 synchronization of both heads is guaranteed by the hard disk programme (Microcode). The parameters of this program are summarized in the production and are individual for every single hard disk.