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Auxiliary memory, also known as auxiliary storage, secondary storage, secondary memory or external memory, is a non-volatile memory (does not lose stored data when the device is powered down) that is not directly accessible by the CPU, because it is not accessed via the input/output channels (it is an external device). In RAM devices (as flash memory) data can be directly deleted or changed.
It is used to store a large amount of data at lesser cost per byte than primary memory; secondary storage is two orders of magnitude less expensive than primary storage.
The most common forms of auxiliary memory devices used in consumer systems are flash memory, optical discs, magnetic disks and magnetic tape . The latest addition to the auxiliary memory family is flash memory. This form is much faster as compared to predecessors, as this form of auxiliary memory does not involve any moving parts. In some laptops, such as Apple's MacBook Air line and MacBook Pro with Retina Display line, solid state drives (SSDs) made from flash memory have replaced magnetic disk based hard disk drives (HDDs) that are found in most laptops. Flash storage chips tend to be significantly more expensive at modestly large capacities compared to HDDs.
- Flash memory: An electronic non-volatile computer storage device that can be electrically erased and reprogrammed, and works without any moving parts. Examples of this are flash drives, memory cards and solid state drives. A version of this is implemented in many Apple notebooks.
- Optical disc: A storage medium from which data is read and written by lasers. Optical disks can store much more data — up to 6 gigabytes more than most portable magnetic media, such as floppies. There are three basic types of optical disks: CD-ROM (read-only), WORM (write-once read-many) & EO (erasable optical disks).
- Magnetic Disk: A magnetic disk is a circular plate constructed of metal or plastic coated with magnetized material. Both sides of the disk are used and several disks may be stacked on one spindle with read/write heads available on each surface. Bits are stored on the magnetized surface in spots along concentric circles called tracks. Tracks are commonly divided into sections called sectors. Disk that are permanently attached and cannot be removed by the occasional user are called hard disks. A disk drive with removable disks is called a floppy disk drive.
- Magnetic tapes: A magnetic tape transport consists of electric, mechanical and electronic components to provide the parts and control mechanism for a magnetic tape unit. The tape itself is a strip of plastic coated with a magnetic recording medium. Bits are recorded as magnetic spots on tape along several tracks. Seven or nine bits are recorded to form a character together with a parity bit. R/W heads are mounted in each track so that data can be recorded and read as a sequence of characters.