Secondary Storage

• Secondary storage (permanent storage) allows computer systems to store large amount fo data instructions and information.
• Compared with memory, secondary storage offers the advantages of non volatility (not losing data), greater capacity and greater economy.
• it cannot be processed directly by the CPU.
• Secondary memory is much slower
• It includes magnetic disk like hard drive and floppy disks optical disks such as CDs and CD-ROMs and magnetic tapes which were the first forms of secondary memory.
• Permanent
• It has huge capacity

Access Methods

• Data can be accessed in either sequential or direct.
• Sequential access means that the data must be accessed in the order in which it is stored.
• Direct access means that the data can be retrieved directly without the need to pass by other data in sequence.

Devices

1. Magnetic tapes: similar tape found in audio and video tapes. Portions of the tape are coated in iron oxide and magnetized to represent bits.
2. Magnetic disks: are coated in iron oxide steel and magnetized to represent bits
3. RAID: Redundant Array of Independent/Inexpensive Disks is a method of storing data in multiple copies of different drives. This process is called disk mirroring and provides an exact copy that protects users. It's a way of storing the same data in different places thus redundantly in multiple hard disks. By placing data on multiple disks, input and output operation can overlap in a balanced way improving performance. It is used for mirroring files and the redundancy helps to maintain two or more copies of the same data on different disks so if one disk fails the system can continue to operate by using the unaffected disk.
4. Optical: A common form of optical disk are compact disk read only memory (CD-ROM). However this form of optical disk can only be read. Optical is faster than magnetic. 
5. DVD: Digital Versatile Disk (DVD) is a 5 inch disk that stores 5.3 GB plus (depends on the type). A DVD only reads information. However recently DVDs have also been developed that read and write.
6. Flash memory: is a silicon computer chip that unlike RAM keeps its memory when the power is shut off. 
 

Recently developments in technology have allowed CD ROM's to be both read and write.