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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.
                   The CPU accesses data stored on a hard disk much faster than it can access data on a CD.


                                     •   External Hard Disks are also available. They are the hard disks that can be
                                        connected to any computer with the help of a USB port.



                    Info Byte

                     The first hard disk was introduced by Seagate in 1979, which could hold 5 MB of data.


                 •  Compact Disc (CD): CD or Compact Disc is a common optical disc. It is made up of light
                   weight plastic material with a chemical coating on one side where data is stored.
                    It is inserted into a CD/DVD drive or CD/DVD writer of a CPU. In CD ROM (Compact Disc Read
                   Only Memory) data can only be read whereas in CD writer, data can be read and written as
                   well. The capacity of 120 mm standard CD is 700 MB.

                   Types of CDs:
                      ◦  CD-ROM  (Compact  Disc Read Only  Memory):  A CD-ROM is a CD that
                      contains data  that  was  written  when  the  CD was  manufactured.  The  data
                      cannot be modified or erased.

                      ◦  CD-R (Compact Disc Recordable): A CD-R can be used to write data by the user only
                      once. This data cannot be changed or erased.
                      ◦  CD-RW (Compact Disc Rewritable): A CD-RW can be used to write data again and again.
                      The data can be erased and modified as many times as required.


                                •   Digital Versatile or Video Disc (DVD): DVD or Digital Versatile/Video Disc is
                                    an optical disc which looks like a CD but has a capacity to store data up to 17.1
                                    GB. 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 a DVD, one has to use a
                                    DVD writer).


                 •  Blu-Ray Disc: Blu-ray Disc can store data of upto 128 GB. It has been developed
                   by the Blu-ray Disc Association (BDA). It is much advanced than CDs or DVDs.
                   Blue-violet laser is used to read or write data in the Blu-ray disc.
                 •   Flash Drive: The Flash Drive is a small portable data storage device integrated with a USB
                   (Universal Serial Bus) connector.
                                  ◦    Pen Drive: It is small in size, handy and is a detachable device from which

                                   data can be read, copied or transferred from one system to another. It is re-
                                   writable and weighs less than 25 gm.

                       Nowadays, pen drives are the most commonly used external storage and data
                      transferring device. It has an attachment slot which can be easily inserted or attached
                      into the computer’s USB (Universal Serial Bus) port.


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                                                                              Computer—Hardware Components
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