SECONDARY STORAGE

SECONDARY STORAGE

Secondary storage (Auxiliary storage) is any storage device designed to retain data and instructions in a relatively permanent form.  Secondary storage is non-volatile meaning that saved data and instructions remain intact when the computer is turned off.   The following are the forms and elements of secondary storage:

-       Diskettes

-       Hard disks

-       Flash memory

-       Compact disks

-       Optical tape

-       Punched cards

-       Magnetic tapes

-        

(a)    Floppy Diskette: This is a thin circular, flexible plastic disk with a magnetic coating enclosed in a square-shaped plastic shell.  A floppy disk drive (FDD) is a device that can read from and write to a floppy disk.  If the computer  has two drives, the first drive is usually designated drive A and then the second B ie   (A:)

  (B:)

            The most widely used floppy is 3.5 inches which might either be high

 density(HD) or Double density(DD).  Before anything can be written on a

new floppy disk or hard disk, it must be formatted.

                                                Fig:  A floppy diskette

            N.B Formatting is the process of preparing a floppy disk or hard disc for

              reading and writing by organizing the disk into storage locations called

            tracks  and sectors. For reading and writing purposes, sectors are grouped

            into Clusters.

CARING FOR DISKETTES

➢    Never expose the diskette to direct sunshine.

➢    Never bend or sit on it in your pocket.

➢    Do not expose it to moisture.

➢    Do not expose it to magnetic field e.g by placing it on top of a monitor or radio.

➢    Never touch the circular plate.

➢    Never pull out the diskette from the drive when light is on.

➢    Never format a disk that is not yours or one that still holds important information.

➢    Keep diskettes in a safe place preferably in a locker in a water proof container.

Advantages of floppy diskettes

➢    Floppy disk is portable and inexpensive.

➢    Data on a floppy can be accessed randomly

➢    Data on a floppy disk can write protected from being changed accidentally.

➢    Floppy can be used to transfer data from one computer to another.

➢    Their storage capacity is quite considerably bigger compared to previous media like punched cards.

Disadvantages

➢    Since they are external, they are vulnerable to harsh conditions of storage like temperatures, moisture, bending, breaking e.t.c which might cause life being unreliable.

➢    Access time of floppy disk is slow (about 84ms).

➢    Data can easily be lost once the magnetic media is exposed to a magnetic field.

(b)   Hard disk/Fixed disk:  This is a metallic media which reside inside the computer for storing computer programs and data. Disk space is measured in Megabytes or Gigabytes.  Typical size is 540mb,  1.2gb, 6gb,  10gb,   20gb,  80gb e.t.c.  The capacity of a hard disk is much higher than that of a floppy diskette.

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Fig: A hard disk

Advantages

➢    Since they reside inside the computer, they can’t easily be stolen or misplaced and data is secure.

➢    The speed and time of data access is very fast and convenient.

➢    Though disks have been known to fail with very long periods of non use, the data life of a hard disk is quite long once in use.

         Disadvantages

➢    Hard disks are susceptible to virus attacks especially in unprotected systems thereby causing vital data loss.

➢    They are not portable since they reside in a computer.

➢    These disks may fail due to violent shaking (vibrations)

➢    They are metallic and expand depending on changes in temperature.  This may cause loss of data.  Use of air-conditioned rooms is therefore recommended.

Comparison between a floppy and hard disk

Hard disk	Floppy disk
1. Has bigger storage than a floppy 2. Fixed with in a system. 3. Has greater speed in saving     information than a floppy. 4. Fixed in a system unit from     environmental hazards. 5.  Organises data in a combination      of tracks called cylinders	1. Low storage 2. Is portable 3.  low storage space 4.  More expose to hazards. 5. Organises data in a single concentric circles called tracks.

N.B Advantages and disadvantages plus definitions can serve as characteristics.

(c)    A compact Disk (CD):  A Compact disk is a flat, round, portable storage medium that is usually 4.75 inches in diameter and less than one twentieth of an inch thick.  Most of the computer software today is supplied on CD’s.  It is capable of storing large amounts of information.

Fig:  A compact CD

The drive designation of a compact disk drive usually follows alphabetically after that of a hard disk i.e if the hard disk is C: then the compact disk is drive D:

     

Variations of Compacts Disks designed for Use with Computers include:

     (i)  CD-ROM (Compact Disk Read Only Memory)

This is a disk that uses the same laser technology as audio CDs for recording music.  A CD Rom can how ever contain graphics, video and sound.  The contents of CD ROMS are written or recorded by the

manufacturer and cannot be modified by the user.         

A CD-ROM drive or CD Rom player is required to read items on a CD ROM.   It can hold up to 650MB of data, instructions and information.

                 

    (ii)  CR-R (Compact disk – Recordable)

This is a technology that allows a user to write on CD using a computer.  CR-R is a “write once, read many” technology.  A user can write on the disk in stages – writing on part of it one time and writing on another part at a later time.  However, the user can write on each part only one and cannot erase it. CD-R software and a CD drive are required in order to write on a CD-R.

Advantages of CD-R

➢    It is more portable than a hard disk.

➢    The storage capacity of a compact disk is very much larger than that of a floppy disk.

➢    The average access time of a compact disk is faster than that of a floppy disk.

   Disadvantages

➢    Some kinds of CDs are read only (CD ROM and DVD ROM)

➢     The average access time of a hard disk is slower than that of a hard disk.

    (iii)  CD-RW (Compact Disc-Re-writable)

This is an erasable compact disk that can be written on multiple times.  A CD-RW acts like a floppy disk or hard disk, allowing data, instructions and information be written onto it multiple times. CD-RW software and a CD-RW drive are required in order to write and re-write on a CD-RW.  One problem with a CD-RW is that they cannot be read by all CR ROM drives.

(iv)  DVD-ROM (Digital Video/Versatile Disk- ROM)

This is an extremely high capacity disk capable of storing from 4.7GB to !7GB.  A DVD Rom drive or DVD player is required to read a DVD ROM.  Although the size and shape of a DVD Rom and a CD ROM are similar, a DVD Rom uses the following techniques to increase its storage capacity.

-       Making the disk denser by packing the pits together.

-       Using two layers of pits, which double the capacity of the disk.

-       Some are double sided, which means that they can be removed and turned over to read the other side.

(v)  DVD-RAM.  Is a recordable and r e-writable versions of DVD Rom which allows items to be erased to be recorded on it multiple times.

(vi) DVD-R. is the recordable and rewritable versions of DVD ROM which can be written once and read (play) for many times.

   (vii) DVD-RW is the recordable and re-writable versions of DVD ROM which can be written and read (play) for many times.  A DVD-RW is similar to a CD-RW except, it has storage capacities up to 4.7GB.

   (viii) Photo CD is a type of CD that contains digital photographic images saved on a photo CD format developed by Eastman Kodak.

CARE FOR COMPACT DISCS

Most manufacturers guarantee that a properly cared for compact disk will last up to 50 years.  Pay attention to the following points for taking care of compact disks.

➢    Always store the compact disk in a jewel box when not in use.

➢    Always hold a compact disk by its edges.

➢    Never touch the under side of the compact disk.

➢    Never stack disks on top of each other.

➢    Never expose the disc to excessive heat or sunlight.

(d)   Magnetic tapes:

These came into use because of the failure of punched cards.  They comprise of magnetically coated ribbon of plastic with magnetic surface onto which data is recorded.  The disadvantage was that, they were bulky and required more storage space.

(e)     Punched cards

Among the first forms of storage devices in computers were the punched cards.  They were flat cards, which consisted of rows of numbers and letters neatly arranged in columns.  To store data, the card was inserted in a computer drive and then a computer punched a series of holes in the rows of characters to represent data stored.  In the read process, a computer made out words in order of the holes.

                                

  Disadvantages

➢    Cards were made out paper and could easily be destroyed by moisture, rats or tear.

➢    They were small rectangular cards of about 5x10cm and only few sentences would be stored on hence limited storage capacity.

➢    Their data life was rather limited because paper depreciates with time.

Other types of storage media include:

-       Zip disk

-       Mo Disk

-       PC Card

-       Smart card

-       Online storage

-       Microfilm and Microfiche.