The Structure of The Hard Disk

The structure of the hard disk is the same as that of a diskette: except in the case of large volumes, different disks are stacked one on top of another and enclosed in a case. A disk is characterised
by the number of platters (also known as cylinders) it has.

The plates of the hard disk.

Each disk is covered with a thin magnetisable film (brown oxide or pure metal vapour).
The plates turn around an axis, generally at a speed of 3600 revolutions per minute.
A read/write head reads data from or writes data to the magnetised surface.

The head sends out a magnetic beam. It never touches the surface and is placed at a distance of 0.0005 mm. The read heads are assembled on a kind of comb. The hard disk assembly is supervised by the controller card.

Characteristics

• Capacity: the memory space capacity, also called storage space, is the volume of data that can be recorded on the hard disk. This is the most important item to be taken into consideration when you purchase a hard disk. Opt for a capacity two or three times greater than you actually expect to use.The unit of measurement is the gigabyte (Gb); that is, a capacity of one thousand million bytes.
• Access time: the time necessary for the read head to find the exact spot where the data are situated. It is of the order of a few milliseconds.
• Latency time: the time for which the computer waits before reading the data. It is a few milliseconds.
• Data transfer speed: the number of data items that can be transmitted from the hard disk to the memory during one unit of time. The unit of measurement is millions of bits per second
  (Mbits/s). The data transfer speed is on average 10 Mbits/s (just over 1.2 Mb per second).
• Interlacing: the number of rotations of the hard disk for reading all the sectors on a track.

Disk drive nomenclature
The floppy disk drives and the hard disks are represented in Windows Explorer by alphabetic letters followed by a colon.

• A: symbolises the first floppy disk drive.
• B: symbolises the second floppy disk drive. If you haven’t got one, the letter B will remain unused.
• C: represents the first hard disk.
• D: represents the second hard disk if present. Each new hard disk has an additional letter assigned to it.
• E: generally represents the CD-ROM drive.