17 November 2010
Understanding articles written about hard disk drives can be confusing to those who are not familiar with the industry's basic terminology. But the terms are actually easy to understand. Following are some of the most often-used hard disk drive terms:
The read/write heads actually float above the spinning platters on a cushion of air created by the spin of the platters.
Platter
Inside the sealed unit of your computer's hard disk drive(s) are one or more platters, or plates, attached to a fixed spindle, looking for all intents and purposes like an old record player with a stack of records waiting to be played. In the case of hard disk drives, however, the platters are not touching each other but are instead separated by a fixed amount of space. Each platter has a top side and a bottom side, and these sides are coated with an easily magnetized substrate. The top side of a platter is Side 0 and the underside is Side 1. When power is applied to the disk drive, the spindle turns at a fixed high rate of speed, thus causing the platters to spin.
Tracks
Each platter contains concentric circles called tracks. Tracks are numbered from the outside in, with the outermost track being track 0 on each side of a platter. So for a hard drive having 12 tracks, track 0 would be the outermost track and track 11 would be the one closest to the spindle.
Cylinders
A cylinder consists of the same track on both sides of all platters. So, using our example of a hard drive having 12 tracks per side on its platters, cylinder 0 would consist of all the outermost tracks, and cylinder 11 would consist of all the innermost tracks.
Sectors
To achieve order in the storage of data on the platters, the platters are electronically divided into pie-shaped wedges called sectors. Sectors are where the actual data is stored. Each sector holds 512 bytes of data and has a unique address, just like a house in a neighbourhood. A sector's address consists of the platter number, the platter's side number, the track number, and the sector number on that track.
Clusters
The 512-byte sectors are commonly grouped into clusters for easier manipulation. One partitioning scheme uses cluster sizes of 4K, while another uses 32K clusters. Some operating systems allow the user to choose the cluster size, even though one particular size will usually be recommended by the operating system manufacturer.
Heads
Heads are the devices used to write and read data onto and from the hard disk drive's platters. Each head is attached to an arm which moves the head into position over a particular sector or cluster, and consists of pretty much nothing more than a tiny coil of wire embedded into a ceramic housing. In Write mode, when the coil is energized in one direction, the head "writes" a 0 onto the platter's substrate beneath it; that is, it polarizes a tiny section of the disk platter in a certain direction. Energize the coil in the other direction, and the head "writes" a 1. In Read mode, the coil senses the polarization of the data bit on the platter as it spins past, and it transfers that information back to the read/write circuitry.
Head Crash
The read/write heads actually float above the spinning platters on a cushion of air created by the spin of the platters. When something happens to cause the heads to overcome the centrifugal force of the platter’s spin, the heads can make contact with the platters, resulting in what is called a head crash or Head-To-Disk-Interference. This is usually catastrophic, as the heads will actually scrape the magnetic substrate right off the platters, destroying data irrevocably. Salvaging data after a head crash is extremely difficult and tedious and, therefore, quite expensive, although a good, professional data recovery company can certainly achieve this.
