Learn in the textbookAuxiliary Storage and Flash Media — HDD, SSD, USB, SD
DefinitionAn HDD is an auxiliary storage device that spins a magnetic disk and reads and writes data with a magnetic head that moves across it.
ExplanationIt is a non-volatile device that keeps its contents when the power is off, and it serves mainly as a PC's auxiliary storage. Every read or write has to wait for the head to move and the platter to turn, which adds mechanical delay. It is cheap per unit of capacity and easy to make large, but its moving parts leave it vulnerable to shock and vibration.
AnalogyThink of a record player. Just as you lower the needle onto a spinning record to find a track, an HDD moves its head to find where the data sits. And like a needle, the head does not take kindly to being jolted.
Memory hookHard Disk means a rigid disk, a name chosen to contrast it with the floppy disk, which used a flexible one.