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. This type of 'soft' bad sector can generally be
fixed by completely erasing the disk (by writing every sector over with a
pattern of zeroes).
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.
Fortunately there are ways to
fix or work around bad sectors on a disk.
Hard disk checking and repair
There are three main methods of
testing and fixing the integrity of your drives and your data. We will look at
some applications that perform these tests a little later in the
Error checking is the process
of scanning the files and folders present on your drive, searching for corrupt
files and inconsistencies. For example, deleted files which the file system
still contains entries for, or files which are referenced more than once. This
can clean up basic file problems which might be causing data issues on your
disk. If you constantly experience errors while performing error checking it
could be a sign that your drive is dying, and bad sectors are proliferating.
Error checking takes very little time to carry out.
surface scan examines each
sector of the disk in detail in order to identify and mark bad sectors. These
areas of the disk can then be marked off-limits to the file system, and spare
sectors used to replace them. Surface scans typically take a large amount of
time to perform, but are the only non-destructive way to map out any bad sectors
on your drive that have developed since you last formatted it.
SMART diagnostics test: A SMART test
queries the internal SMART (Self-Monitoring, Analysis and Reporting Technology) information of
your drive for its current status. This serves as
a test of your drive's current physical well being. We'll get into the
benefits of using SMART next, but first, what are the warning signs of a
failing hard drive?