A bad sector is a sector of the disk to which
data cannot be written. This may be due to a number of reasons,
but to your operating system all that matters is that it can
no longer use that portion of the disk. If the disk has yet
to be used, or is being reformatted, bad sectors are not really
an issue. All recent operating systems map a drive's bad sectors
and avoid them while formatting it for use.
Modern hard drives are manufactured with 'spare' sectors
which are only used to replace space lost due to bad sectors.
This allows a drive to automatically compensate for any slight
manufacturing defects which may have resulted in the drive
having bad sectors 'out of the box.' When formatting a drive,
bad sectors are detected and these 'spare' sectors are used
to replace them as necessary.
Where bad sectors become an issue is when previously usable
sectors become damaged due to disk error. Any data on these
sectors is in danger of being corrupted or destroyed. The
operating system has no way of automatically compensating
for this loss, potentially leading to data corruption on a
larger scale and system instability if the affected data was
crucial.
Bad sectors can be created by software errors, or slight
errors in the read/write heads which cause the data laid down
on that particular sector to be completely illegible, and
thus appear as bad to the operating system.
Bad sectors created by a mechanical error have a tendency
to breed more bad sectors quickly. Since the margin for error
inside a working disk is so slim, any surface damage or debris
created by a mechanical failure or 'head-crash' will quickly
cause more damage as it interferes with the read/write heads.
When formatted, a drive with 'hard' bad sectors like these
will use the spare sectors to as much as possible replace
the existing bad sectors as detailed above. Of course, if
the bad sectors are still spreading due to loose particles
or surface errors in the disk, the disk will still be unstable.
At this point, the only option is to salvage as much data
as possible from the drive before it dies completely.
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