Hard disk

From Ganfyd

A form of semi-permanent storage in the computer, consisting of ferromagnetic spinning platters within a tightly sealed outer casing. It should be distinguished from a solid state disk, although either is critical for data storage in stand alone computers, and either type can fail without warning.

The failure rate of a usual hard disk is about 1 in 3 in five years. This tells you not that much about the frequency, which is non-linear and will be device dependent but is the reason why you need backup of any critical information.