There is hardly any desktop, laptop or server today that does not use one or more hard disks. Even high-end and very powerful machines such as mainframe computers and supercomputers use an array of hundreds of hard disks to create a massive bank of digital storage.

Hard disk drives are so versatile and useful that, besides being the standard form of data storage for computers, even consumer electronic devices such as mobile phones, digital video disk (DVD) players and MP3 players have begin to come equipped with micro hard drives.

Hard disks are popular because they are quite inexpensive and store data in a non-volatile form that can safely remain intact for years unless there is some damage to the hardware. When computers and other digital machines are switched off, the hard disks ensure that the data is made available to them the next time they are put to use.

When hard disks were originally invented over half a century ago, they were huge devices about 20 inches in diameter. In spite of the size, they had a capacity of a few MB which at that time was considered revolutionary. The scientists decided to call the devices as “hard” drives to contrast them with the soft and limp floppy drives. The former stored data on a hard metal surface while the latter used a thin film of plastic as the data-recording surface. While floppy disks existed separately from their drives and could be removed by the user, this was not the case with the hard disks which remained fixed inside a computer CPU. Therefore, they also came to be called as fixed disk drives (FDDs).

At the fundamental level, a hard disk is not much different from a floppy drive or cassette tape. All of them use the same principles of storing data. There is a recording surface made of plastic, glass or metal which is coated with a thin layer of magnetic paint. A read / write head carrying an electro magnet modifies the orientation of magnetic particles to record digital data. Such data can be written, erased and rewritten tens of thousands of times without any problem. When the disk is without power, the magnetic flux patterns on the recording surface continue to exist independently. Such non-volatile data remains the same regardless of how many times the device is powered on or off, unless it is modified by the read / write head.

Hard disks have two major benefits over other storage devices – lightening-fast random access and enormous capacities. No other storage media available in the market offers both of these features together. Though hard disks started with a puny capacity, today their storage capacity has become formidable. The commonly available disks offer a capacity of 80 GB to 160 GB, though those with a capacity of 500 GB are also available. One manufacturer recently launched a drive with a capacity of an unbelievable one terabyte.

Hard disks offer random data access. This means that their read / write head can extract any data file instantly from any part of the recording surface and supply it to the CPU for processing. This becomes possible due to the fact that in a hard disk, the read / write head as well as the platters on which data is stored are both non-static. The head located at the end of an actuator arm can hover over any part of the platter surface. The platters, in turn, spin at a very rapid rate. The commonly available hard drives have a platter speed of either 5,400 or 7,200 revolutions per minute (RPM). As a rule, the higher the RPM, the better it is because the faster the hard disk can access data.

Though hard disk drives are made as rugged as possible by the manufacturers, they still remain quite delicate machines that are susceptible to hardware damage if not handled properly. They have a normal life of about 25,000 hours of use but it is common for them to crash anytime due to some defect. This happens when the read / write head that hovers a microscopic distance over the platters comes crashing down on the surface. Hard disks can also be damaged due to impact trauma, water or fire damage, lightening strike and failure of inner components such as controller card, circuit board, spindle motor ball bearings and so on.

Hard disks are very sensitive to dust and become unusable the moment they are opened in an ordinary room environment. The only way to carry out repairs is by disassembling them in a clean room. The latter has restricted access and is equipped with special filters that remove even microscopic air-borne dust particles and other foreign matter from the environment. The disks are repaired and reassembled in the clean room and handed over to the customer for continued use.