Hard Disc Drives


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Hard disk drives are similar to the floppy disk drives with some major differences.  First the disk rotates approximately 20 times faster than the floppy disk at a speed of 7200 to 10,000 rpm as opposed to 360 revelations per seconds the read and writes head is flying one.  That is the head is attached to the slight airfoil like an airplane wing.  The third major difference is that the disk is made of rigid oxide coated aluminum instead of flexible Mylar plastic.




The hard disk drive is a sealed unit that holds most of the data in a computer system.  A hard disk drive is made up of many components, and many are various types suds as st506 interface, ESDI (Enhanced Small Device Interface), IDE (Integrated Device Electronics), SCSI (Small Computer System Interface) etc., in these entire however, the basic mechanical components are generally the same they are:


1.      Disk platters

2.      Read and Write heads

3.      Head Actuator (Stepper or Voice coil)

4.      Spindle Motor

5.      Logic Board

6.      Connectors etc.


The platters spindle motor; heads and head actuator mechanism are usually contained in a sealed champers called hard disk assembly (HDA). The HDA generally forms single assembles, since if can be opened in a dust free, environment only.




The typical hard disk has one or more platters or disks of generally 5.25" or 3.5" diameter.  Cold high capacity drives used 8" and 14" platters also.  All these platters are mounted on a single shaft, and driven (at about 7,200rpm to 10,000 rpm) by the spindle motor. The platters are about 0.05" thick and coated with a magnetically (aluminum) retentive substance or media, which is actually responsible for the storing the data.  The thickness of this media is about 30 microns.




The hard disk drives have read and write head for each platters surface. Depending upon the derive model number, there may be 4 to 15 heads, all which are mounted on common movement mechanical each head is mounted on an arm that is spring loaded to force the heads on to the platters.  At rest, the heads are in contact with the platter.  But, when the drive is spinning out full speed, air pressure, it formed below the read and write head, which lifts it away from the platters.




The electro mechanical, that moves the read and write head assembly on the platter surface, is called the actuator.  Two types of actuators one commonly used.  In a stepper motor type actuator, this movement is achieved by a stepper motor, which uses a metal band to more the head assembly7 from outer edge of platter to its inner edge, and back.   In a voice coil type actuator the similar movement of the head in achieved using a voice coil. It must be noted that while voice coil has a spring, which brings the read and write head automatically to the park position on power down.  In a stepper type actuator the heads hare to be parked by a suitable disk park program before moving the computer.




Together the platters rotate as a unit on a shaft, called the spindle.  Typically the short connects directly to a spindle motor that spins the entire assembly.  Most hard disks use servo-controlled spindle motors, which constantly monitor their own speed using optical or magnetic sensors and automatically compensate for any variation.  Typically hard disk platters spin at about 7200rpm to 10,000 rpm.




All the electronics in the disk drives is located on the logical boards.  This electronics controls the spindle motor, head actuator, read and write head signals from or to the head, the signal interface between the hard disk controller and drive etc.




Platters of the hard disk are made from an aluminum alloy. The aluminum serves as substance to which a magnetic medium is affixed either with a binder or mechanically.  In addition to that it in coated writer oxide, by the method of electroplating.




Concentric circles on a disk that are numbered from the outside starting of 0 to most floppy disk have either 40 or 80 tracks per side, which hard disk have many more some as many as 1000.



A sector is that part of a track lying with in what could be farmed a "pick slice" of the disk.



Tracks of the same dos number on each platter form a cylinder.  Cylinder made of same tracks on different platters.



A clusters is a set of configures sectors, which a hard disk may have four or more sectors per clusters.







I) DISK CAPACITY: -The capacity of hard disk depends upon the total number of cylinders, heads, sectors per track and number of bytes per sectors.  For instance, if a drive has 1024 cylinders, 124 heads, 17 sectors per track, the total capacity will be

1024*12*17*512=102.0MB. In the present day disk with capacity ranging 2GB to 80GB are commonly available.




When data is requested from the hard disk by the computer, the controller, signals, the hard disk to move the head to the specified track where the data is stored.  This operation is called seek operation moving a head take a finite amount of time is known as head setting time.




The average access time is calculated based on the track to track seek time.  The seek time is the duration taken by the head to move from one track to another.



It is time taken for a sector to reach a correctly position head, which is directly related to the speed of the rotation of the disk.



Before a hard disk can be used to store data it must under go three types of preparations.

1.      Low-level formatting (physical format)

2.      Partitioning

3.      High-level formatting (Logical format)


            1. Low level formatting

                        The disk controller performs the low level formatting and it is done by the hard disk manufactures.

            2.  Partitioning

                        This is done through the dos external command fdisk.  This program divides the disks into one or more partitions.  If determines the operating system to be loaded in different partitions of the hard disk.



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