Another classification: Primary, secondary, and tertiary storage.
Primary storage : the fastest storage media, such as cash and main memory.
Secondary (or on-line) storage : the next level of the hierarchy, e.g., magnetic disks.
Tertiary (or off-line) storage : magnetic tapes and optical disk juke boxes.
Volatility of storage . Volatile storage loses its contents when the power is removed. Without power backup, data in the volatile storage (the part of the hierarchy from main memory up) must be written to nonvolatile storage for safekeeping.