Introduction of Magnetic Disk

  • Magnetic disk consists of Magnetic Oxide materials.
  • A modern computer uses two types of magnetic memory or disks: –
    1. Magnetic Disks:
      • Magnetic disks are the most common form of non-volatile secondary storage because they provide fast access and high storage capacities at a reasonable cost.
      • Examples are – Hard disks and Floppy disks.
    2. Magnetic Tapes :
      • This is another type of magnetic memory but less used. eg- Tape Recorder and Video recorder Cassettes.
  • Hard Disk Drive(HDD) Unit : 
    • A disk is mounted (or stacked) on the disk drive, which has the motor that rotates it.
    • Hard disks together with read/write heads, access mechanisms, and driving motors constitute a complete single unit called hard-disk-drive (HDD) unit. The whole unit is fixed.

Hard Disk


  • The largest size of inexpensive, non-volatile, secondary memory of a computer is known as Hard Disk.


  • Its access time is 5-20 ms (millisecond).
  • Hard disks are online storage devices i.e. the hard disk works when the computer is ON.
  • Disk Controller :
    • Several disk drives (such as C, D, E, F, etc.) in a computer are connected and controlled by a structure called a disk controller.
    • The controller converts instructions received from the computer (software) to electrical signals to operate disks.
    • The Disk controller accepts commands from the computer and positions the read/write head of the specified disk for reading or writing purposes.
    • For reading or writing operations on a disk pack, the computer must have the clear-cut information of Disk Address Format which consists of drive number(3 bits), cylinder number(13 bits), surface number(4 bits), and sector number(8 bits). Here, the drive number must be specified (compulsory), because a controller normally controls more than one drive at a time. The above example of disk address format is for a disk controller of 8 drives, each disk pack having 250 cylinders, 12 surfaces, and 256 sectors.
  • Data Access on Magnetic Disk :
    • Magnetic disks are semi-random devices i.e. a track on a disk is selected in random mode, but data is written to or read from a sector of that track serially.
    • To access information on a disk, the disk address of the desired data has to be specified. The disk address includes the surface number, track number, and sector number.
    • Information is always written into the disk from the beginning in a sector (of a particular disk surface and track) and can be read only from the track beginning.
    • As soon as the read/write command is received by the disk controller, the read/write heads of a disk are first positioned onto the specified track number (or cylinder) by moving the arm assembly in the proper direction.

Structure of Hard disk

  • Magnetic disk drives contain metal disks that are coated on both sides with an iron oxide recording material.
  • Several disks are mounted together in a magnetic disk on a vertical shaft which typically rotates the disks at speeds of 3600 to 7600 revolutions per minute (rpm).
  • A magnetic disk is a surface device, i.e. it stores data on its each surface of a platter.
  • Cylinder :
    • A set of corresponding tracks on all the (n-1) disk surfaces, at a given radial distance, is called a cylinder of the disk.
    • The concept of the cylinder is very important because data stored on the same cylinder can be retrieved much faster than if it were distributed among different cylinders.
  • Platter :
    • A Hard disk is a collection of several refined, circular, thin plastic materials called platters.
    • It can not be removed or inserted into an HDD unit later.
    • Some magnetic disks have a single platter e.g. floppy disk.
    • To increase the storage capacity of a hard disk, several platters are mounted (stacked) vertically, normally at a distance of an inch. This is known as a disk pack or multi-platter configuration.
  • Heads/Conducting Coil :
    • There is an electromagnetic read/write head positioned by access arms between the slightly separated disks to read and write data on concentric, circular tracks.
    • The read/write head reads data from the disk and writes data to the disk.
    • A single head is associated with each surface of the platter for read/write operation.
  • Tracks :
    • Data are recorded on circular areas of the disk called tracks, in the form of tiny magnetized spots to form the binary digits of common computer codes.
    • Thousands of bytes can be recorded on each track, and there are several hundred data tracks on each disk surface, which provides billions of storage positions for our software and data.
    • Each side of the platter surface contains several circular concentric areas called tracks.
    • The number of tracks on a disk range up to 800.
  • Sectors :
    • Each track is divided into several small areas called sectors (normally 10-100).
    • These sectors can be either fixed or variable-length sectors.
    • The division of track into equal-sized blocks or pages is set by the operating system during disk formatting.
    • The number of bytes stored in each sector is kept the same.
    • The numbers may vary but there are often 200 or more ranging up to 800 is sectors per track.
  • Magnetic disks are semi-random devices. A track on a disk is selected in a random fashion, but data is written to or serially read from a sector.
  • HDD contains magnetic disks, access arms, and read/write heads into a sealed, air-filtered box/enclosure using a technique called as Winchester technique.
  • Winchester disk is another name for “hard disk drive”.
  • The Winchester disk was developed by IBM at Winchester, New York State of USA that had 30MB of fixed storage and 30MB of removable storage and hence the first model number was given as 3030.
  • The name Winchester disks is mainly due to the first model number of the disk being given as 3030, which was also the model number of the well-known Winchester Rifle popular in the Wild West. Although modern disk drives are faster and hold more data the basic technology is the same, so Winchester has become synonymous with hard disk.
  • Thus Winchester disk is a sealed “hard disk” having a rotation speed typically 7200 rpm.
  • A Winchester disk has 5000 to 10,000 concentric tracks per centimeter and about 100,000 bits per centimeter around the circumference.

Magnetic Disk : Hard Disk Track Portion


  • It is very robust.


  • It has comparatively slower performance than other memory media especially when the disk is full.


  • There are two types of magnetic disk arrangements –

(i) Removable Disk Cartridge :

    • Removable disk devices are popular because they are transportable and can be used as backup copies of our data.

(ii) Fixed Disk Unit :


  • HDD(Hard Disk Drives) stores programs, data, operating systems, compilers, assemblers, application programs, etc. permanently. Thus, HDD is used to store or retrieve information quickly, easily, and safely anytime on the computer.

Magnetic Tapes

  • A Magnetic tape is a sequential access type secondary storage device.
  • It is mainly used for backups in servers, workstations, and large computers.
  • The main advantages of magnetic tapes are that they are cheaper since they are removable from the drive and also they provide unlimited storage capacity (20 GB to 150 GB/more).
  • The read/write heads of magnetic tape drives record data in the form of magnetized spots on the iron oxide coating of the plastic tape.
  • Magnetic tape devices include tape reels and cartridges in mainframes and midrange systems, and small cassettes or cartridges for PCs.
  • The main drawback of magnetic tapes is that they store information sequentially. A file or some particular information stored on a magnetic tape cannot be accessed directly on a random basis as is possible in the case of hard disks or floppy disks.
  • These devices are slower, but due to their low cost, they are still widely used for massive data warehouses and other business storage requirements.
  • The storage capacity of a tape is measured by multiplying its length and data recording density. Data recording density is the amount of data that can be stored on a given length of tape.

Storage Capacity = Data Recording Density * Length

  • Advantages :
    • It has a very Large storage capacity.
    • These disks are durable, robust, and rewriteable.
    • They are inexpensive.
  • Disadvantages :
    • Data cannot be accessed immediately/randomly.
    • Requires tape drive and third-party software to read/write the information.
    • Tape drives for large-capacity magnetic tapes can be very expensive.


Categories: Memory Unit


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