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Contents > Concepts
Chapter 3. CONCEPTS
This chapter describes some basic concepts that might help when
unerasing data.
A hard disk is a sealed unit containing a number of platters in a
stack. Hard disks may be mounted in a horizontal or a vertical
position. In this description, the hard drive is mounted horizontally.
Electromagnetic read/write heads are positioned above and below
each platter. As the platters spin, the drive heads move in toward the
center surface and out toward the edge. In this way, the drive heads
can reach the entire surface of each platter.
3.1.1. Making Tracks
On a hard disk, data is stored in thin, concentric
bands. A drive head, while in one position can read or write a
circular ring, or band called a track. There can be more than a
thousand tracks on a 3.5-inch hard disk. Sections within each track
are called sectors. A sector is the smallest physical storage unit on
a disk, and is almost always 512 bytes (0.5 KB) in size.
The figure below shows a hard disk with two platters.
Figure 3-1 Parts of a Hard Drive
The structure of older hard drives (i.e. prior to Windows 95) will
refer to a cylinder/ head/ sector notation. A cylinder is formed while
all drive heads are in the same position on the disk. The tracks,
stacked on top of each other form a cylinder. This scheme is slowly
being eliminated with modern hard drives. All new disks use a
translation factor to make their actual hardware layout appear
continuous, as this is the way that operating systems from Windows 95
onward like to work.
To the operating system of a computer, tracks are logical rather
than physical in structure, and are established when the disk is
low-level formatted. Tracks are numbered, starting at 0 (the outermost
edge of the disk), and going up to the highest numbered track,
typically 1023, (close to the center). Similarly, there are 1,024
cylinders (numbered from 0 to 1023) on a hard disk.
The stack of platters rotate at a constant speed. The drive head,
while positioned close to the center of the disk reads from a surface
that is passing by more slowly than the surface at the outer edges of
the disk. To compensate for this physical difference, tracks near the
outside of the disk are less-densely populated with data than the
tracks near the center of the disk. The result of the different data
density is that the same amount of data can be read over the same
period of time, from any drive head position.
The disk space is filled with data according to a standard plan.
One side of one platter contains space reserved for hardware
track-positioning information and is not available to the operating
system. Thus, a disk assembly containing two platters has three sides
available for data. Track-positioning data is written to the disk
during assembly at the factory. The system disk controller reads this
data to place the drive heads in the correct sector position.
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