The hardware used for storing and retrieving data is called secondary storage, or simply storage. Data stored on a storage medium must be read into memory before the CPU can access it. A backup is a copy of data held on a separate storage medium for security purposes. Backups should be performed regularly. Information systems need to store data because memory is limited in size and volatile. Volatile means that when the computer is switched off, or the power supply is interrupted, the contents of memory are lost. Secondary storage provides a non-volatile and more permanent storage area than memory. The hardware involved in storing and retrieving includes devices such as magnetic disks, magnetic tape, optical disks and flash memory.