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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
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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
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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.
Basics of Information Technology 27

