This Unit is about Storage devices in computers.

A computer uses primary memory and secondary memory to store data.

(i) PRIMARY MEMORY such as RAM provides a small amount of temporary storage area for the data and instructions required by the CPU for processing.

(ii) SECONDARY MEMORY is used by Computer systems to store larger amounts of data, and information more permanently than allowed with primary memory.

When a user issues a command to start an application program, the operating system locates the program in secondary storage and loads it into primary memory.

In this Unit, we shall focus on secondary memory

Definition of Terminologies

A Storage medium is the physical material on which a computer keeps data. There is a variety of storage media available.

Capacity is the number of bytes (characters) a storage medium can hold.

A Storage Device reads and writes data to and from a storage medium.

Reading is the process in which a storage device transfers data, from a storage medium into memory.

Writing is the process in which a storage device transfers data from memory to a storage medium (saving).

Access time, is a measure of the amount of time it takes a storage device to locate an item on a storage medium.

Transfer rate is the speed with which data, instructions, and information move to and from a device.

Transfer rates for storage are stated in KBps (kilobytes per second)

Categories of Secondary Storage Media

There is a wide variety of storage devices in the following categories.

(A) Magnetic media, or

(B) Optical media and

(C) Other Types of Storage Media such as

Punched Cards

USB flash drive

Flash memory cards

Photographic film

Microfilm and Microfiche

(A) Magnetic media

Magnetic storage media represent data as magnetic spots on the tape or disk, with a magnetized spot representing a 1 bit and the absence of such a spot representing a 0 bit.

Common examples of magnetic media include:

i.Magnetic tape

ii.Floppy disk,

iii.Zip and Jaz disks

iv.Hard disks

(i) Magnetic tape

Magnetic tape is a magnetically coated ribbon of plastic capable of storing large amounts of data and information at a low cost.

Tape storage requires sequential access, i.e. data must be accessed in the order in which it is stored.

If the computer is to read data from the middle of a tape, all the tape before the desired piece of data must be passed over consecutively.

Today, magnetic tape storage is no longer used for routine processing.

Demerits of Magnetic tapes

  • Random data access is not possible.
  • Magnetic Tape Data storage has a limited shelf life of about 2 years only.

(ii) Floppy disk (diskette)

A floppy disk, or diskette, is a portable, inexpensive storage medium that consists of a thin circular, flexible plastic disk with a magnetic costing enclosed in a square-shaped plastic shell

A standard floppy disk is 3.5-inches wide and has storage capacities up to 1.44 MB.

A floppy disk drive is a device that can read from and write to a floppy disk.

Before you can write on a new disk, it must be formatted.

Formatting is the process of preparing a disk for reading and writing by organizing the disk into storage locations called tracks and sectors

Care for diskettes

  • A floppy disk should not be exposed to heat, cold, magnetic fields and dust.
  • Never leave diskettes in the disk drive. Diskettes should be rolled up and stored in pencil holders.
  • Diskettes should not be inserted or removed from the drive while the red light is flashing.
  • Avoid touching the inner magnetic strip of the diskette.

Advantages of Floppy Disks

  • Floppy diskettes are portable
  • Floppy diskettes are cheap
  • Random Data Access on a diskette is possible
  • A floppy diskette can be write- protected from being changed.

Disadvantages of Floppy Disks

  • Floppy diskettes are not reliable – they need to handled with a lot of care, else risk loosing data.
  • Floppy diskettes are not durable.
  • Data Access time is relatively slow.
  • They have limited Storage capacity (only 1.44MB)

(iii) Zip drive

The Zip drive is a medium-capacity removable disk storage system with capacities of 100 MB to 750 MB that was introduced by Iomega in late 1994.

However it was never popular enough.

Zip drives fell out of favor for mass portable storage during the early 2000s due to emergence of much better USB flash drives CDs, and DVDs.

(iv) The Jaz drive

Similar the the Zip drive, The Jaz drive was a removable disk storage system, introduced by the Iomega company in 1995.

The Jaz disks were originally released with a 1 GB capacity .

The rising popularity and decreasing price of CDs and DVDs greatly hurt the success of the Jaz Drive and the Jaz line was ultimately discontinued in 2002.

Zip and Jaz drives with media.

(v) Hard disks

A hard disk, also called hard drive, usually consists of several inflexible, circular metal platters coated with magnetic oxide that can be magnetized to represent data.

The entire device is enclosed in an air-tight, sealed case to protect it from contamination.

Hard disks come in a variety of sizes.

In most personal computers, the hard disk is housed inside the system unit.

Current personal computer hard disks have storage capacities from 160 GB to 1 TB and more.

A track is a narrow recording band that forms a full circle on the surface of the disk. The tracks are further divided into sectors.

A cluster, also called allocation unit, consists of two to eight sectors of space used to store data.

Sometimes, a sector has a fault and cannot store data. When you format a disk, the operating system marks these bad sectors as unusable.

A cylinder is the vertical section of track through all platters on a hard disk.

A read/write head is the mechanism that reads items and writes items in the drive as it barely touches the disk’s recording surface.

When a read/write head does accidentally touch the platter surface, it is called a head crash and all data is destroyed.

Thus, it is crucial that you back up your hard disk regularly.

A backup is a duplicate of a file, program, or disk placed on a separate storage medium that you can use in case the original is lost, damaged, or destroyed.

Hard disk interfaces

The hard disk interface defines the physical means by which the hard disk connects to the rest of the computer.

There are many disk interfaces;

External hard disk interfaces include:



Internal hard disk interfaces Include:

SATA, (Serial Advanced Technology Attachment)

EIDE, (Enhanced Integrated Drive Electronics)

SCSI, (Small Computer System Interface) and

SAS (Serial-attached SCSI)

Hard disk Access Speed

Depending on the type of hard disk, transfer rates range from 15 MBps to 320 MBps.

Access time for today’s hard disks ranges from approximately 3 to 12 milliseconds.

Hard disks improve their access time by using disk caching.

Disk cache, sometimes called a buffer, consists of a memory chips on a hard disk that stores frequently accessed items such as data, instructions, and information. It is similar to Memory Cache

Types of Hard Disks

  • An Internal hard disk is fixed in the system unit and usually stores the operating system required for the computer to work.
  • An external hard disk is a separate free-standing hard disk that connects with a cable to a USB port or FireWire port.
  • A removable hard disk is a hard disk that you insert and remove from either a dock or a drive.
  • An Internet hard drive, also called online storage, is a service on the Web that provides storage to computer users, usually for a minimal monthly fee.

Hard disk Durability

Most manufacturers guarantee their hard disks to last somewhere between three and five years. Many last much longer with proper care and handling.

To prevent the loss of items store on a hard disk, you regularly should perform preventive maintenance such as defragmenting or scanning the disk for errors.

(B) Optical Media

Optical storage refers to recording of data by making marks in a pattern that can be read back with the aid of light, usually a beam of laser light.

The reflected light is converted into a series of bits that the computer can process.

An optical disc is a flat, round, portable storage medium made of metal, plastic, and lacquer that is written and read by a laser.

Optical discs used in personal computers are 4.75 inches in diameter.

Smaller computers and devices use mini discs that have a diameter of 3 inches or less.

Care for Optical Disks

The following should be done for the safety of data on Optical disks:

  • Do not expose the disc to excessive heat or sunlight
  • Do not eat, smoke or drink near a disc.
  • Do not stack disks.
  • Do not touch the underside of the disk.
  • Always store the disc in a jewel box when not in use
  • Always hold a disc by its edges.
  • Do not drop the disk to the ground.
  • Don’t bend the disk.

Categories of Optical Disks

Two general categories are CDs and DVDs, with DVDs having a much greater storage capacity than CDs.

Examples of Optical Disks include:

CD-ROM (compact disc read-only memory)

The contents of standard CD-ROMs are written by the manufacturer and only can be read and used. A typical CD-ROM can hold from 650 MB to 1GB of data, but most hold 700MB.

Picture CD

A Picture CD is a compact disc that only contains digital photographic images saved in the jpg file format.

You can purchase Picture CDs that already contain pictures.

A Picture CD is a multisession disc, which means you can write additional data to the disc at a later time.

CD-R (compact disc-recordable)

CD-R (compact disc-recordable) is a technology that allows you to write on a compact disc using your own computer’s CD-R drive.

Once you have recorded the CD-R, you can read from it as many times as you desire.

A CD-R is a multisession optical disc which allows you to write on part of the disc at one time and another part at a later time.

However, you cannot erase the disc’s contents.

CD-RW (compact disc-rewritable)

A CD-RW (compact disc-rewritable) is an erasable multisession disc that you can write on multiple times.

Reliability of the disc tends to drop, however, with each successive rewrite.

To write on a CD-RW, you must have a CD-RW drive and CD-RW software.

A CD-RW drive has a lower read and write speed as compared to CD-Rs

Magneto-optical (MO) disk

This is a hybrid disk, that combines the best features of magnetic and optical disk technologies.

It has the erase and rewrite capabilities of magnetic disks, but it also has the very high-volume density capabilities of optical disks.

MO disks are not popular because they are too expensive, and not as reliable as magnetic media.


DVD-ROM (digital video disc-ROM). A DVD-ROM is an extremely high capacity compact disc capable of storing from 4.7 GB to 17GB.

In order to read a DVD-ROM, you must have a DVD-ROM drive, which can also read CDROMs.

DVDs are also available in a variety of recordable and rewritable versions and formats such as DVD-R and DVD+R DVD+RW, DVD+RE, and DVD+RAM.

High Capacity DVD formats

A Blu-ray Dics-ROM (BD-ROM) has storage capacities of up to 100 GB.

The HD (high-density) DVD-ROM has storage capacities up to 60 GB.

A mini-DVD that has grown in popularity is the UMD (Universal Media Disc), which can store up to 1.8 GB of games, movies, or music.

Other Types of Storage Media

1. Punched Cards

A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions.

They were used through the 20th century in unit record machines for input, processing, and data storage.

Early digital computers used punched cards, often prepared using keypunch machines, as the primary medium for input and storage of both computer programs and data.

Punched cards

An 80-column punched card of the type most widely used in the 20th century.

USB flash drive

A USB flash drive is a flash memory storage device that plugs in a USB port on a computer

USB flash drives are convenient for mobile users because they are small and lightweight enough to be transported in a pocket.

Current USB flash drives have data storage capacities ranging from 256 MB to 64 GB.

USB flash drives have become the mobile user’s primary portable storage device, making the floppy disk nearly outdated.

Flash memory cards

Flash memory cards are a type of solid-state media, which means they consist entirely of electronic components and contain no moving parts.

Common types of flash memory cards include Memory Stick, CompactFlash (CF), SmartMedia, miscroSD, miniSD, xD, Picture Card, etc.

They are commonly used in electronic devices such as digital cameras and mobile phones. They are tiny, re-recordable, and able to retain data without power.

Depending on the device, manufacturers claim these storage media can last from 10 to 100 years.

Photographic film

Photographic film is a sheet of plastic such as polyester coated with a light sensitive emulsion, that is used to record and store photographs.

Normal film is shipped in small canisters (boxes) that protect it from the light.

When exposed to light, it forms an invisible image.

Chemical processes can then be applied to the film to create a visible image, in a process called film developing.

A normal photographic film can hold up to 40 pictures.

Microfilm and Microfiche

These are media used to store microscopic images of documents on roll or sheet film.

The images are recorded onto the film using a device called a computer output microfilm recorder.

The stored images are so small they can be read only with a microfilm or microfiche reader.

Microfilm and microfiche have the longest life of any storage medium.

Libraries use these media to store back issues of newspapers, magazines, and genealogy records.




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