Let us look at the construction of the hard disk.
All hard disks consist of thin platters with a magnetic coating.
They rotate quite fast inside a metal container. They register bits from the magnetic coating, which races past them. On the illustration below, you see a hard disk with three platters. The synchronous movement of these arms is performed by an electro-mechanical system called the head actuator. The hard disk data can only be attained via one head at a time.