A hard disk can be either internal or external and one can use it to save files. The parts of a hard disk are the disk case, spindle, disk platter, actuator and the read-write arm.

Ferrite or cobaltite crystals are within a hard disk.

The hard disk actuator moves the heads so that they can read different tracks. There are two different types of actuators: stepper motors and voice coils. Most modern mechanical hard drives use voice coils. SSD drives make even this obsolete.

There is no fixing the actuator, replace it with a new one.

The "master/slave" designation is necessary for the disk controller. It has little to do with the dual boot.