Secondary storage

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In computer storage, secondary storage, or external memory, is computer memory that is not directly accessible to the central processing unit of a computer, requiring the use of computer's input/output channels. Secondary storage is used to store data that is not in active use. Secondary storage is usually slower than primary storage, or internal memory, but also almost always has higher storage capacity and is non-volatile, which makes it perfect for the preservation of stored information in an event of power loss.

Storage devices in this category include:

• CD, CD-R, CD-RW
• DVD
• Flash memory
• Floppy disk
• Zip Drive
• Hard disk drive
• Magnetic tape
• Paper tape
• Punch card
• RAM disk
• External Hard Drive
• Blu-Ray
• Thumb Drive

Almost all storage devices are either magnetic or optic. A well known exception is provided by flash memories (used in digital audio players, usb stick memories, etc.) which uses a different approach.

The secondary storage is often formatted according to a filesystem format, such as ext3 or ntfs which provides the abstraction necessary to organize data into files and directories, providing also additional information (called metadata) describing the owner of a certain file, the access time, the access permissions, and other information.