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The surface of each disk is marked with invisible circular tracks. The data is stored in the form of magnetic
                 spots on disk.

                 l    Hard Disk: It is the most widely used internal storage magnetic disk. It is made up of spindles
                      of smooth metallic plates coated with thin layer of magnetic material. Each plate of a hard disk
                      is called a platter. Each platter has two (read/write) heads, one on each side. Over the years hard
                      disks  have been available in various storage  capacities. e.g.,  20 MB in earlier  days to 160 GB,
                      320 GB, 500 GB, 1 TB, 5 TB, 6 TB, 12 TB, 16 TB and more in recent times.

                      Do you know?


                      Hard Disk is also known as Winchester Disk.


                 Optical Disc

                 Optical disc is commonly used as an external storage device in which the laser beam helps to read data
                 from the flat surface of the optical disc.

                      CD: CD or Compact Disc is a common optical disc. It is made up of light weight
                 l
                      plastic material with a chemical coating on one side where the data is stored.

                      It is inserted into a CD/DVD drive or CD/DVD writer of a CPU. CD ROM (Compact
                      Disc Read Only Memory) reads the data whereas in CD writer, data can be  written
                      as well. The capacity of 120 mm standard CD is 700 MB.


                                        DVD: DVD or Digital Versatile/Video Disc is an optical disc which looks like a CD
                                   l
                                        but has a capacity to store data up to 17.1 GB. DVD is the merging of video and
                                        audio in a common format. To read information stored in a DVD, it has to be
                                        inserted into a DVD ROM or DVD Writer or DVD player (to copy information on
                                        to a DVD, one has to use a DVD writer).

                 l    Blu-ray Disc: Blu-ray disc can store data upto 128 GB. It has been developed
                      by the Blu-ray Disc Association (BDA).
                      It is more advanced than CDs or DVDs. To read or write data on a Blu-ray disc,
                      a Blue violet laser is used.

                      Do you know?

                      v    Diameter of a CD, DVD or Blu-ray disc is 120 mm. DVD can also be of 80 mm diameter that can hold
                         data upto 1.5 GB.
                      v   CDs can be used or played in a DVD-ROM or a DVD player but DVD cannot be operated in a CD
                         player.
                      v  A combo drive is a device which contains all the features of a CD-Writer and a DVD ROM.



                 Flash Memory
                 Flash Memory is a small sized external storage device that is commonly used in digital cameras, mobile
                 phones, MP3 players, etc. It can be plugged through USB or Fire wire port of a computer to fetch or
                 transfer data to computer.


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