Hard Disc - Online Article

The role of hard disc

The hard drive is the component which is used to permanently store data, as opposed to RAM, which is erased whenever the computer is restarted, which is why the term mass storage device is sometimes used to refer to hard drives.

The hard drive is connected to the motherboard using a hard drive controller which acts as an interface between the processor and the hard drive. The hard drive controller manages the drives linked to it, interprets commands sent by the processor and routes them to the drive in question. Hard drives are generally grouped by interface as follows:

  • IDE
  • SCSI
  • Serial ATA

When the USB standard appeared, external cases which could connect a hard drive using a USB port were released, making hard drives easy to install and increasing storage capacity for macking backups. These are called external hard drives, as opposed to internal hard drives which are plugged directly into the motherboard; still, they are the same disks, even though they are connected to the computer using a case plugged into a USB port.


A hard drive is made up of not just one, but several rigid metal, glass, or ceramic disks, stacked very close to one another and called platters.

Hard drive

The disks turn very quickly around an axle (currently several thousand revolutions per minute) in a counter-clockwise direction. A computer works in binart mode, meaning that the data is stored in the form of 0s and 1s (called bits). Hard drives hold millions of these bits, stored very close to one another on a fine magntic layer a few microns thick, which is covered by a protective film.

They are read and written using read heads located on both sides of the platters. These heads are electromagnets which raise and lower themselves in order to read or write data. The read heads are only a few microns from the surface, separated by a layer of air created by the rotation of the disks, which generates a wind of about 250km/h (150 mph)! What's more, these disks are laterally mobile, so that the heads can sweep across their entire surface.

Structure of a hard drive (read heads)

However, the heads are linked to one another and only one of them can read or write at a given moment. The term cylinder is used to refer to all the data stored vertically on each of the disks.

This entire precision mechanism is contained within a fully airtight case, as the smallest particle can degrade the disk's surface. This is why hard drives are closed shut with seals, and the warning "Warranty void if removed", as only hard drive manufacturers can open them (in particle-free "cleanrooms").

How it works

The read/write heads are said to be "inductive", meaning that they can generate a magnetic field. This is especially important in writing: The heads, by creating positive or negative fields, polarise the disk surface in a very tiny area, so that when they are read afterwards, the polarity reversal completes a circuit with the read head, which is then transformed by an analog-digital converter (ADC) into a 0 or 1 which can be understood by the computer.

Tracks on a hard drive

The heads start writing data from the edge of the disk (track 0), then move onward towards the centre. The data is organised in concentric circles called "tracks", which are created by low-level formatting.

The tracks are separated into areas (between two radii) called sectors, containing data (generally at least 512 octets per sector).

Hard drive sectors

The term cylinder refers to all data found on the same track of different platters (i.e. above and below one another), as this forms a "cylinder" of data.

Hard drive cylinders

Finally, the term clusters (also called allocation units) refers to minimum area that a file can take up on the hard drive. An operating system uses blocks, which are in fact groups of sectors (between 1 and 16 sectors). A small file may occupy multiple sectors (a cluster).

On old hard drives, addressing was done physically, by defining the position of the date from the coordinates Cylinder/Head/Sector (CHS).

Hard Disk Drive Interfaces De-mystified

There are several types of interfaces available, Which one do you choose? What do these different letters mean? How do I know which one is correct for my PC?

Technology is advancing at breathtaking pace. As soon as a new technology emerges it is named by three or four letters and expect the consumer to understand everything behind the technology and adopt it without any questions. Interface is the connector technology and more accurately it the technology of transferring data to and from the disk drives. A brief history/guide to the various technologies are described below:

IDE - Integrated Drive Electronics

This is a popular term, it really should be called ATA - Advanced Technology Attachment. One of the earliest types. Also known as ATA-1. Data transfer rate of about 8Mbps (8 million bits per second).

EIDE - Enhanced IDE

Also known as ATA-2 or fast ATA, offered speeds of upto 16Mbps.


This was a minor revision to ATA-2.


Also known as Ultra ATA/33, Ultra DMA, ATA-33 and DMA-33. Transfer speeds of upto 33Mbps. This was revised to offer support for CD ROM drives and was known as ATAPI (ATA Packet Interface).


Also known as Ultra ATA/66, ATA-66 and Ultra DMA-66. Transfer speeds of upto 66Mbps.


Also known as Ultra ATA/100, ATA-100 and Ultra DMA-100. Transfer speeds of upto 100Mbps.


Also known as Ultra ATA-133, ATA-133 and Ultra DMA-133. Offers speeds of upto 133Mbps and is expected to be the last update to the ATA standard.

SATA - Serial ATA

This is one of the latest and popular developments. The high speed serialized AT attachment. Currently SATA offers ransfer speeds of upto 150Mbps. However, this technology can offer upto 600Mbps with further development. The next stage is increasing the speed to 300Mbps.

PATA - Parallel ATA

Another new technology? NO. Since the introduction of Serial ATA, the ATA technology is now referred to as the Parallel ATA or PATA.

Some of the earlier ATA developments like IDE and EIDE are obsolete, but even today some drives are categorized as IDE or EIDE. They are bound to be of the newer ATA types, but due to the popularity of IDE and EIDE the name has stuck. Since Serial ATA gaining prominence, changing IDE to ATA would only add to the confusion.

Searial ATA II or SATA II does not exist. It is a common misconception and anyone labelling their drive as SATA II with 300Mbps(3.0Gbps) would be incorrect. The higher speed may be available in the near future, but not so at the time of writing this guide.

So, How Do I Choose a Hard Disk drive?

Firstly, you need to find out the type of disk drive your PC mainboeard can accept. Most of the latest ones can accept both ATA and Serial ATA. If this is the case then choose a Serial ATA disk drive. However, if you are on a very tight budget then you may be able to save a little by choosing an ATA/IDE disk drive.

If your computer is quite old and you need a hard disk, then there is no need to panic. ATA/IDE disk drives are backward compatible. They will work without any problems, but maybe at a reduced performance level. Always refer to your mainboard manual and the operating system before purchasing your drive. Some older operating systems cannot work with disk drives greater than 40GB.

Choosing the size or capacity of the disk drive is dependent upon the size of your wallet. It also depends on what sort of work you will be doing on your PC. If it is for holding your music collection, videos and photos then stick with a 300Gb hard disk. If it is for general work then anywhere from 80 - 160Gb will be fine. Always check the prices. Sometimes, for an additional 10 - 11 you may be able to get an additional 50Gb of space.

I have not emphasized, SCSI - Small Computer System Interface, which is another type interface. These disk drives are more commonly used on servers and workstations. due to the relative cost involved they never became popular for PCs. Unless your PC is equipped to accept SCSI, it is not easy to install SCSI disk drives. You will need to get an additional adapter/card to connect SCSI disk drives. The prices may still be relatively high as well.

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Shamit Kumar Tomar on 2008-07-27 15:42:32 wrote,

It would be better if you had given some images of hard disc too.