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Hard Disk | Hard Drive


Hard Disk a secondary storage device used to store data on computers, mobiles and laptops. Introduced by IBM in the year 1956, a hard disk is a non-volatile, random access data storage device.

Introduced by IBM in the year 1956, a hard disk is a non-volatile, random access data storage device. The inside of a hard disk consists of a disk like plates on which data is stored and retrieved. The data on these disks are read/write by magnetic heads called the read head and write head. From the moment when the hard disk is started these disks start rotating and the head moves to and fro to read/write the data on these disks.

But in those early days hard disks were expensive and when a user brought a computer, he/she had to order a hard disk separately i.e. the hard disk was not included in the package. It is estimated that a single megabyte cost around $15,000 and that these hard disks were huge in size. Only those who could afford it would purchase it and it was a luxury good rather than a need based good. But as advancement in technology took place the hard disk became a common data storage device and along with it the cost and size also reduced. In the early days the hard disk had a maximum space of about 10 MB and today it has a maximum space of 1TB (1000 GB).


A hard disk consists of a spindle that rotates the plates on which the data is recorded. These data disks are accompanied by two headers as data can be written or read on both side of the disk. The nature of these disks is ridged and not flexible like a CD disk. These disks are made of aluminum alloy or glass which are non magnetic in nature and on top they are coated with a layer of magnetic material on which the data is stored and this layer is protected with a coat of carbon. The speed of these disks is 3000 Rotations Per Minute (RPM) to 15,000 RPM, where high speed hard disks are used for servers and low speed hard disks are used for home or medium office use. The data on these disks is written and read by headers which are magnetic in nature and it moves very close to the magnetic surface of the disks.

Hard Disk


The capacity of a hard disk is defined in Megabytes (MB), Gigabytes (GB) and Terabytes (TB). Today’s hard disks come with a capacity of gigabytes and terabytes making megabyte an obsolete capacity. One megabyte consists of 1000,000 bytes, one gigabytes consists of 1024 megabytes and one terabyte consists of 1024 gigabytes.

Form Factor:

The earlier hard disks were the size of a washing machine, but as time passed the personal computer came equipped with a hard disk which had a form factor or size of about 8 inches, followed by a 5.25 inch size in the year 1980 which was introduced by Seagate, this size is still in use in many desktop computers. This size was followed by a 2.5 inch hard disk which is used with laptops and music players. The form factor reduces in size to 0.85 inch announced by Toshiba in the year 2004. Currently the form factors in use are 3.5 inch with a maximum capacity of 3 TB, 2.5 inch with a maximum capacity of 1.5 TB and 1.8 inch with a maximum capacity of 320GB.

Disk Drive Performance:

The most important thing about a hard disk is the time it takes to read or write data on the disk. It means that the time taken by the hard disk to retrieve the required data by the end user. The disk performance depends on factors like Access Time and Data Transfer Time.

Access Time:

It is the measure of time that a rotating disk takes to before the drive can actually transfer the data. And this depends on the mechanical nature of the rotation of the disks and the moving headers. The Access Time is further divided in two parts called the Seek Time and the Latency Time.

Seek Time:

It’s the time taken by the header to reach the area on the disk where the data has to be written or read. Currently the average seek time is 3milliseconds for server based hard drives while common desktop computers have about 9millisecond seek time.


It is the time required to bring the required disk sector under the read/write header so that it can be accessed and this depends on the speed of rotation which is measured in RPM. Like seek time Latency is also measured in milliseconds, so a hard disk with an RPM of 4,200 would take 7.14ms to bring the required disk sector under the header while a hard disk with an RPM of 15,000 would take 2ms to do the same thing.

Data Transfer Time:

This part deals with both the internal rate of transfer and external rate of transfer. Disk Transfer Time can be measured by writing a huge file on the disk and then reading it. A typical 7,200 RPM hard disk would have a data transfer rate of about 1030 Mbits/Sec.


Today’s hard disks are less expensive and more fast, apart from this manufacturers are making hard disks which are shock proof so that they can resist shock making them ideal for mobile or laptop use. Thought progress in speed and timing is made, hard disks are still not considered as the ultimate medium to store data as these hard disk are vulnerable which in turn can result in data loss and thus users are generally advised to have a back up of their data someplace else too.

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