Secondary Memory and three its three types

Secondary Memory:

Secondary memory or storage can store a ruler of information permanently and lies outside the CPU. It provides computer the means of storing and processing large quantities of data for longer periods of time and that is not currently being passed. Secondary storage is also call Auxiliary storage or external memory. It is non volutile.

Purpose of Secondary storage:

Auxiliary storage is used to store.
Input data and programs.
Output results of processing.
Auxiliary storage is where us put last years results of ur school, addresses for old students, programs you may or may not each use, data you entered yesterday everything that is not being used right now.
Secondary memory(or Secondary Storage) is the slowest and cheapest form of memory.
It can not be processed directly by the CPU. It must first be copied in to primary storage (RAM).
Types of Secondary Storage Devices:
Secondary memory devices includes magnetic storage devices like floppy disks, hard disks and magnetic tapes(the first forms of secondary memory) and optical disks such as CDs and CD-ROMS.
Magnetic Storage Devices: 
Magnetic storage devices are designed in such a way that their surfaces are coted with millions of thing from particles. So that data can be stored on them each of these particles can act as a magnet in magnetic field when subjected to an electromagnet. A read/write head is use to access data from these devices.
Storing Data in Magnetic Storage Devices:
The read write head of a hard disk device, floppy disk device and tap device contain electromagnets. Which generate magnetic field in the from on the storage medium as the head posses owr the disk or tape. The read write head record strings of is and of by alterneting the direction of current in the electro-magnets.
    There are following magnetic storage devices.
(1)Floppy Disk   (2)Hard Disk    (3)Magnetic Tape
Optical Storage Devices:
Optical storage techniques make use of the user beams to write and read data which can consist of the graphics, audio clips, or video image at magnetic disk. a single optical disk of the type called CD-ROM can hold up to about too megabytes of data. This works out to about 269,000 pages of test, or more than 7500 photos or graphics or 20 hours of speech or 77 minutes of video. Although some disks are used strictly for data storage many combine, visuals and sound. In the principal type of optical disk technology a high power loser beam is used to represent data by burning tiny pits into the surface of a hard plastic disk. To read the data, a now powers loser light scans the disk surface: pillet area are not reflected and are interpreted as 0 bits: Smooth areas, called land are reflected and are interpreted as 1 bits. Because the pits are very tiny much more data can be represented than is possible in the same amount of space on a magnetic disk whether flexible or hard.
Many types of optical disks are available for computer users. Among them the commonly available optical disks are:

1. CD-ROM DISKS
2. CD-R DISKS
3. CD-RW DISKS 
4. DVD-ROM DISKS