Every hard disc contains one or more flat discs that are used to actually hold the data in the drive. These discs are called platters. They are composed of four key layers: a substrate material that forms the bulk of the platter and gives it structure and rigidity, a magnetic media coating which actually holds the magnetic impulses that represent the data, a non-magnetic protective layer that separates the media from the head and a lubricant layer that reduces the friction between the media and the head.

Hard disks follow the so-called bathtub curve model of failures: they have a relatively high rate of "infant mortality" (or early failure), a period of very low failures for several years, and then a gradual reduction in reliability as they reach the end of their useful life.

To ensure that those who rely on hard drives do not continue to use them as they enter the period of increased failure that results from component wear-out, every hard disk has defined as one of its specifications the manufacturer's intended service life for the product. In real world life, the service life of hard disc drives is very often higher than the stated one which tends to be very conservative.