Hard Disk
A hard disk or a hard disk drive or an HDD or a hard drive is a storage device for storing data typicaly used with computer system. It stored the data on a magnetic surface layered onto hard disk platters. Hard disk are non-volatile in nature. This means that they do not lose the information they contain when the electrical sypply (or power) is withdrawn or turned off.
History of Hard Disks
IBM 350 Disk File, made by Reynold Johnson in 1955 was the first hard disk. A slow drive, it had a capacity to hold 5 million characters. This hard disk used a single head to read data from each of the 24 platters.
It was not before 1961, that separate heads for each platters were used. The Winchester disk system, introduced in 1973 and now an industry standard was made by IBM and was the first to make use of a sealed head/disk assembly.
Hard disks were very delicate which prevented them from being used in an industrial environment. They were also large in size and expensive at the same time. Another problem was that of high power requirements. It was not until after 1980, that they were used with microcomputers when the first 5.25-inch drive, the ST-506 was made by Seagate. It could hold 5 MB of data.
Hard disks were usually sold by OEMs as a part of a larger product, rather than the manufacturer selling them seperately. This trend changed in the 1990s when, by 1995, hard disks were available for separate retail sale. Inspite of the popularity of hard disks, they were still not part of the original configuration in some computers including the Apple Macs as late as 1998, making it necessary to attach external storage.
In terms of capacity, there has been a huge increase since the first time hard disks were introduced. Starting off with as less as 5 MB of storage and 20 MB being considered as large (large enough, sometimes), today’s hard disks are available with various storage capacities, ranging from 40 GBs to 750 GBs.
Basic working of a Hard Disk
A hard disk is made of round platters that are magnetic in nature placed one over the other with some space between them. These platters have a corresponding read/write head that access them. Hence, each platter has a head assigned to it. Each platter of a hard disk is divided into circular paths called tracks which are concentric. Each of the tracks is further divided into arcs of equal radius called sectors. The actual data is stored in these sectors.
Each platter rotates at a certain speed with the help of the motor in the center. The speeds at which these platters rotate is measured in ‘Rotations per mintue’ or ‘rpm’ and range from 3600 to 10000. The head is placed at the end of an ‘arm’ that brings it right above the respective platter. This arm is moved in a fast and precise manner using a single-way motor at its other end. The head typically ‘flies’ very close above the platter (without actually touching it). When data is to be written (or read), the platters revolve and the head creates a magnetic field in the space between the platter and itself. It is through this that the data is read or written onto the hard disk. Since the platters are rotating, it is important to determine where the requested data is located in order to retrieve it as fast as possible.
Hard Disk types
The type of hard disk refers to the interface or connection used by it to communicate with a computer. Some hard disk types in use today are as follows:
* IDE or ATA
IDE stands for Integrated Drive Electronics and is also called Advanced Technology Attachment (ATA) or Parallel ATA. This interface is capable of accessing only one drive on the same channel at a time. If more than one drive has been installed, one would be the ‘master’ and rest all would act as ‘slaves’.
* SCSI
Standing for Small Computer System Interface, SCSI has been around since the mid-90s. Allowing for cable lengths of up to 12 meters, it is typically used for Hard Disks, though it can also be used for other types of drives such as an optical drive.
* SATA
SATA, introduced in 2003, stands for Serial ATA. It features high data transfer rates and is the most advanced interface available today. The SATA interface eliminates the Master/Slave architecture for installing more than one drive on the same computer system. It is very fast as compared to its predecessor, Parallel ATA (PATA or simply, ATA), with a data transfer rate of over 1 Gb while the ATA had only a 50Mb rate.