On a hard disk, data is stored in the magnetic coating of the disk. The so-called head, held by an actor arm, is used to write and read data. Data is organized on a disk in cylinders, tracks and sectors.
Cylinders are concentric tracks on the surface of the disk.
A track is divided into sectors. A hard disk has a head on each side of a disk. All hard disks have reserved sectors, which are used automatically by the drive logic if there is a defect in the media. You may need to cool a 7200-rpm disk with an extra fan, or its life would be much shorter. The rotation speed is constant.