The earliest mention of data storage would probably be during the 1800's when punch cards were used to provide input to calculators and other machines. During the years, many different methods of storing data have lead to the use of magnetic storage. The primary goal has basically been the same. To store computer data and instructions in the form of some number scheme such as binary numbers and to be easily and quickly accessible, reliable and cost effective.
In the 1940's, vacuum tubes were used for storage until tape drives started to replace punch cards. The density of its recording was 556 bits per inch. The fact is, dirt would get into the read/write heads and contaminate them and after a while it would cause the tapes to stop. As a result, continuous cleaning of the heads was required. After that came the magnetic drums. The magnetic drums were considered the "primary memeory" of most computer systems. As the drums rotated along it horizontal axis, the read/write heads accessed data from the magnetic outer surface. The data word under the read/write head(s) could be accessed as the drum rotated. The drum continued to rotate after an instruction was fetched and the word ready to be accessed by the read/write head(s) was usually many words away. This of course, caused a delay and even though the magnetic drum was an advancement, it was often very slow.
The first commercial electronic computer appeared in 1951 and many companies had to settle with leasing a computer because they were expensive. In 1957, for example, IBM introduced its RAMAC 350. This beast required 50 24-inch disks to store five megabytes of data and cost 7,000 dollars per megabyte per year to lease.
The IBM PC/XT which was introduced in 1983, was one of the first PC's that had a fixed hard disk drive. Around that time the maximum storage capacity was 10MB for 5.25 inch full height drives. In 1995 the stated maximum was around 10gigabytes for a small 3.5 inch one-half height drives.
The only constant in the data storage industry is change and any piece of machinery for a computer that works is obsolete, or so it seems like now-a-days. During the year of 1996, 106 million drives were sold and the year of 1997 saw an increase by 18% selling 125 million units. The drives being sold in 1998 are expected to increase 12%, to 140 million units. The cost per Mbytes is expected to continue its decline to 1 cent to 2 cents per Mbyte by the year 2000. (In 1990, the average cost of one Mbyte was a little less than one dollar.)
A hard disk consists of rigid, circular disk-shaped platters. The disks are non-bendable, fixed hard disk drives. Basically, all drives are fixed and not removable. However, there are some removable platter hard disk drives available. These drives are not standard, high in cost and not very reliable. Each platter, fixed or removable, requires two read/write heads, one for each side.
Hard disk drives used to be called Winchester drives. The name was derived back in the 1960's (another reference said 1973) when IBM develoed a 30MB high-speed hard disk drive of removable platter storage and 30MB of fixed-platter storage. The 30-30 drive soon received the nickname Winchester after the Winchester 30-30 reifle.
Data is organized on a disk in cylinders, tracks and sectors. Cylinders are concentric tracks on the surface of the disk. A track is divided into several numbered divisoins know as sectors with the outside of the disk having more space for sectors than the inner parts. A disk track is too large to manage effectively as a single storage unti because many disk tracks can store 50,000 or more bytes of data. Sectors are represented as slices on the track. Each disk sector is 512 bytes in size. (that statement is almost true, each sector occupies 571 bytes of which only 512 bytes are used for user data. The rest are used for secotr header and tailer. This may vary from drive to drive.) hard drives have multiple platters and are double sided so data can be stored on both sides of the platter. All the heads are mounted on a common carrier device or rack and because of this the heads move in and out in unison across the disk.
Magnetic drives operate by using electromagnetism. When an electrical current flows through conductors, a magnetic field is generated around it. This magnetic field then can influence magnetic material in the field. The polarity of this field is reversed when the direction of the flow of the electric current is reversed. An electric motor exerts pulling and pushing forces on magnets attached to a rotating shaft, by the use of electromagnetism.
The read/write heads in disk drives are U-shaped pieces of conductive material. An electric current can pass through this because the U-shaped object is wrapped with coils of wire. A magnetic field in the drive head is generated when the disk drive logic passes a current through these coils. The voltage, in essence, can be switched in polarity very quickly because the heads are electromagnets. The read/write head can change the magnetic charge of the particles to represent 1's or 0's.
When magnetic energy passes from the head to the storage device, it writes. When magnetic entergy passes from the device to the head, it reads. When the head has a magnetic field generated within it, the field jumps the gap at the end of the U-shaped head. Since a magnetic field can pass through a conductor easier than air, the field bends through the medium and uses the disk media directrly below it as the least resistant path to the other side of the gap. During the passing of the field through the media, it polarizes the magnetic particles and aligns the field. The polarity of the magnetic media and the field's polarity are based on the direction of the flow of electric current throught the coils.
Hard disks' platters consists of material such as aluminum or glass on which is layered with a magnetizable material which is usually a form of iron oxide and other added elements. As the disk rotates below the drive head, the head can lay a magnetic flux (a flux is a magnetic field that has a specific direction) over a region of data. A flux reversal is a polarity change of the alignment of magnetic particles on the surface of a disk. This is caused when the electrical current flow through the coils in the head is reversed and so is the magnetic-field polarity in the head. Flux reversals are placed, by the drive head, on a disk to record data. For each bit written, reversals are placed on a disk in areas know as cells. The encoding method (such as Modified Frequency Modulation (MFM) Run Length Limited (RLL)) is used to produce a pattern of flux reversals with the bit cells used to store a data bit or bits. Voltage is applied to the head during the write process. As the polarity of the voltage changes, the magnetic fields' polarity being recorded also changes. At this point the flux transmission are written.
During the read process, the head acts like a flux transition detector. It emits pulses whenever it crosses a transition. In other words, the signal is zero volts unless a positive or negative transition is detected with the results in a positive or negative pulse. A pulse only appears when the head is passing over flux transitions.
By passing an electrical current through an electromagnet (the drive head) causes a magnetic field and this is how data is recorded on a disk. passing the head over the surface of the disk is how data is read. When a change in the stored magnetic field is encountered by the head, a weak electrical current is generated that indicates the absence or presence of flux transitors in the originally recorded signal.
Even though hard drives may vary, the basic components that make up the hard drive are the same. These basic components are as follows: Disk platters, read/write heads, head actuator mechanism, spindle motor, logic board, cable and connectors, configuration items such as jumpers or switches and sometimes a bezel.
