Secondary Memory

This type of memory is also known as external memory or non-volatile. It is slower than main memory. These are used for storing data/information permanently.

CPU directly does not access these memories. Instead they are accessed via input routines. Content of secondary memories are first transferred to main memory and then CPU can access it. For example, disk, CD-ROM, DVD. 

Hard disk:- A hard drive fits inside a computer case and is firmly attached with the use of braces and screws to prevent it from being jarred as it spins. Typically is spins at 5,400 at 15,000 RPM.

Advantages:

• Large storage capacity.
• Stares and retrieves data much faster than a floppy disk or CD-ROM.
• Data is not lost when you switch off the computer.  
• Usually fixed inside the computer so cannot get mislaid.
• Cheap on a cost per megabyte compared to other storage media.
• Hard disk can be replaced and upgraded as necessary.
• Can have two hard disk in machine one can act as a mirror of the other and create a back up copy.          

Disadvantages:

• Hard disk eventually fails which stops the computer from working.
• Regular head crashes can damage the surface of the disk leading to loss of data that sector.
• The disk is fixed inside the computer and cannot easily be transferred to another computer.

Magnetic tape:  Magnetic tape is a recording medium consisting of a thin tape with a coating of a fine magnetic material used for recording analog or digital data.

Optical disk:   Optical disk is electronic data storage medium from which data is read and written to by using a low- powered laser beam. It is flat, circular, plastic or glass disk on which is stored in the form of light and dark pits.

CD:  CD is an abbreviation compact disk and is a form of data storage that can transfers data up to the speed of 7800 KB/s. A standard 20 mm CD holds up to 700MB of data, or about 70 minutes of audio there are two types of CD; CD-ROM and CD-RW, CD-ROM, is stand for CD-Read only memory and they function the same way read only memory does. CD-RW stands for CD-Re writable; these disks can be erased and rewritten at any time.

DVD: DVD is an abbreviation of digital versatile and is an optical disc storage media format that can be used for data storage. The DVD support disks with capacities of 4.7 GB to 17GB and access rate 600 KB ps. to 13 MB ps.

BLU ray disk: BLU ray is an optical disc format designed to display high definition video and store large amounts of data.

BLU-Ray is the successor to DVD.

BLU-Ray’s standard storage capacity is enough to storage a continuous back up copy of most people’s hard drives on a single disc. Initially, the formed had a 27 GB (gigabyte) single sided capacity and 50GB on dual layer discs.

FLASH DRIVE: A flash drive is a small external storage device, typically the size of a human thumb that consists of flash memory. USB flash drives are removable and rewritable reads and write to flash memory.