Data and programs are stored in main memory, as random access memory (RAM), before and after processing by the central processing unit (CPU). However, RAM is volatile —its contents disappear when a computer's power is turned off. So what does one do with data that will be reused, stored on a long-term basis, or is simply too large to fit into the main memory of a computer? Mechanical storage devices, called secondary storage or external storage, are used to store data externally. Secondary storage is non-volatile—that is, data and programs are permanent.

Types of Storage Devices

There are many secondary storage devices, including magnetic drums, magnetic tapes, magnetic disks, and optical disks. These devices vary with respect to their versatility, durability, capacity, and speed.

Magnetic Drums.

These are very early high-speed, direct access storage devices used in the 1950s and 1960s. The magnetic...