Due to volatility and limited size of a computer’s main memory, most computers have additional memory devices called mass storage (or secondary storage) systems, including magnetic disks, CDs, DVDs, magnetic tapes, and flash drives (which will be discuss on next article). The advantage of mass storage systems over main memory include less volatility, large storage capacities, low cost, and in many cases, the ability to remove the storage medium from the machine for archival purposes.
The term on-line and off-line are often used to describe devices that can be either attached to or detached from a machine. On-line means that the device or information is connected and readily available to the machine without human intervention. Off-line means that human intervention is required before the device or information can be accessed by the machine-perhaps because the device must be turned on, or the medium holding the information must be inserted into some mechanism.
A major disadvantage of mass storage systems is that they typically require mechanical motion and therefore require significantly more time to store and retrieve data than a machine’s main memory, where all activities are performed electronically.
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