1. Magnetic Tape
Magnetic tape is the most popular storage medium for large data, which are sequentially accessed and processed. The magnetic tape medium is a plastic ribbon, which is ½” or ¼” wide and 50 to 2400 feet long. It is coated with a magnetically recording material, such as iron oxide. This type of device is still used for large data backup because these are cheaper than other devices. Like audio or videotape, the magnetic tape used in computer system which can also erased and reused indefinitely. Old data on a tape are automatically erased, as new data are recorded in the same area. Tapes for computer are similar to the tapes used to store music. Accessing data on tapes however is much slower than accessing data on disk.
Fig: Magnetic Tape
The features of magnetic tape are:
o Inexpensive storage device
o Can store a large amount of data
o Easy to carry or transport
o Not suitable for random access data
o Slow access device
o Needs dust prevention, as dust can harm the tape
o Suitable for back-up storage or archiving
2. Magnetic Disk
Magnetic disk is a circular disk which is coated with magnetic material. The disk rotates with very high speed inside the computer drive. Data is stored on both the surface of the disk. Magnetic disks are most popular for direct access storage device. Each disk consists of a number of indivisible concentric circles called tracks. Information recorded on tracks of a disk surface in the form of magnetic spots.
Fig: Track and sectors of disk
The features of magnetic disk are:
o Cheap storage device
o Can store a large amount of data
o Easy to carry or transport
o Suitable for frequently read/write data
o Fast access device
o More reliable storage device
o To be prevented from dust, as the read/write head flies over the disk. Any dust particle in between can corrupt the disk
Floppy disk, hard disk and zip disk are the different types of magnetic disks:-
a) Hard Disk:
The hard disk is direct-access storage medium with a rigid magnetic disk. The disk is divided into number of tracks and each tracks are divided into sectors.
A hard disk uses round, flat disks called platters, coated on both sides with a special media material designed to store information in the form of magnetic patterns. The platters are mounted by cutting a hole in the center and stacking them onto a spindle. The platters are rotated at very high speed, driven by a special spindle motor connected to the spindle. Each platters consists of READ/WRITE head to read and write data. The disks are rotated at very high speed (usually around 7200RPM-revolution per minute.)
Fig: Parts of hard disk
b) Floppy Disk:
Floppy disk is a round, flat piece of flexible plastic coated with magnetic oxide. These are small removal, media storage device. The data is recorded on thin plastic film. Floppy disk is a soft magnetic disk and it is a thin piece of flexible plastic called floppy disk, or also called floppy Diskette .They are removable disks.
A floppy disk is inserted inside the computer the floppy drive when needed. These are different sizes of floppy disk 5.25” or 3.5” having capacity 1.2MB and 1,44MB respectively.
Fig: Floppy disk
c) Zip Disk:
· They are high-capacity removable disk and drive.
· They have the speed and capacity of hard disk and portability of floppy disk.
· Zip disk are of the same size as floppy disk, i.e., 3–½ inch but have a much higher capacity than the floppy disk.
· Zip disk and drive were made by Iomega Corp. It comes as a complete unit—disk, drive, connection cable, power cord and operating system. It can be connected to the computer system externally using a parallel chord or SCSI cable.
·Their capacity ranges from 100 MB to 750 MB. They can be used to store large files, audio and video data.
Fig: Zip disk
3. Optical Disk
An optical disk is random access storage medium. It is made up glass. Optical disks uses light technology where laser beam is projected and reflected light is observed. As compared to magnetic tape and disk, optical disk is relatively new secondary storage medium. An optical disk storage system consists of a rotating disk which is coated with a thin metal or some material that is highly reflective. This type of disk uses laser beam technology for READ/WRITE data. Due to the use of laser beam technology, optical disks are also known as laser disk or optical laser disks. Unlike magnetic disks, which have several concentric tracks, an optical disk has one long track, which starts at the outer edge and spirals inwards to the center. The optical disk has no mechanical R/W arm movements.
Advantages of optical disks:
a) The cost-per-bit of storage for optical disks is very low, because of their low cost.
b) The use of a single spiral track makes optical disks an ideal storage medium for reading large blocks of sequential data such as music.
c) Optical disks have no mechanical read/write, which makes optical disks a more reliable storage medium than magnetic tapes or disks.
d) Due to their compact size and light weight, optical disks are easy to handle, store and port from one place to another.
Limitation:
-The data access speed for optical disks is slower than magnetic disks.
- Optical disk requires a more complicated drive mechanism than magnetic disks.
Types of optical disks:
a) CD-ROM (Compact Disk - Read Only Memory): The data is written onto the CDROM disk before it is sold and cannot be changed by the user. CD-ROMs are used for applications such as distributing software, digital videos or multimedia products.
b) CD-R (Compact Disk - Recordable): A CD-R disk is blank when it is supplied. The user can write data to it just once. After data has been written to the disk it cannot be changed. CD-Rs are often used for making permanent backups of data and distributing software when only a small number of copies are required.
c) CD-RW (Compact Disk - Rewriteable): CD-RW disks can be read from and written to.
DVDs
d) DVD-ROM (Digital Versatile Disk - Read Only Memory): DVD disks are able to store much more data than CD disks. The DVD standard includes disk capacities up to 30Gb. DVD-ROM disks can be read from but cannot be written to.
e) DVD-RAM (Digital Versatile Disk - Random Access Memory): DVD-RAM disks have all of the benefits of DVD-ROM disks and can be written to as well. These very high capacity disks are ideal for producing backups. Because of their high capacity, DVD disks are used to store high quality video such as complete movies.
Magneto-Optical Disk
- Magneto-optical disks use laser beam to read data and magnetic field to write data to disk.
- These are optical disks where data can be written, erased and re-written.
- They are expensive and outdated. They were used during the mid-1990s. They have now been replaced by CD-RW and DVD-R.
