A HDD is a mechanical form of storage, using electromagnetism to store and write data.

Mechanical drives are very prone to data lose and breaking when moved around, due to their mechanical nature.

They consist of two main components, an actuator (Consisting of an "Arm" that does the reading and writing), and a platter on which the data is stored.

Data is arranged in concentric (Non spiral) tracks around the platter and are further divided up into segments which run from the center of the pllatter to the outside (Think slices of pizza).

When reading or writing, the arm moves out to the correct segment and then the platter spins so that the arm can get to the correct segment. At this point the arm can either receive the bits of data on the HDD or send out electric signals to re-write the bits on the disk.