The earliest form of rotating magnetic storage device, used in some of the first computers at a time when random-access store was volatile, bulky, and expensive. The drum therefore formed the main memory of some of these machines, the random-access stores being used only as registers. Although random-access store developed rapidly it was still relatively expensive and the drum was retained as a local backing store on some computers. Magnetic disk, when introduced, took over a large part of the backing store function. Drums remained in use however on certain systems that required faster access than was generally provided by disk, but today they are obsolete.

A magnetic drum consists of a cylinder whose curved surface is coated with a suitable recording medium, either metal or iron oxide. This surface is divided into tracks on which data is recorded.