Bad Sectors On Hard Disk Drives: Preventing Data Loss

A sector is an allocation unit of a hard disk drive, when you look at the label of your hard disk you may find the letters LBA (which stands for Logical Block Addresses), this will be followed by a numerical figure. This figure, in current drives, will reach hundreds of millions (or even billions dependent on your definition of a billion!). Every sector on the hard disk drive has a unique consecutive LBA number starting at zero for the first sector on your disk.

During the lifetime of a hard disk drive, one or more of these sectors may become bad or unreadable, this is known in the data recovery industry as either media degradation or simply "bad sectors".

Bad Sectors are areas of a hard disk drive platter that are not able to be properly read from or written to. There can be many reasons for this problem, imperfections on the platter surface that result from the manufacturing process can become apparent over time, physical wear and tear can cause unreadable areas as can external influences such as shock, vibration and environmental electrostatic issues etc.

Why are they a problem?

The thing with bad sectors is that if you are aware that your drive has them, then you are almost always in the midst of a gradual decline in the reliability of the drive as a whole. A modern drive is designed to deal with bad sectors that arise during the lifetime of the drive without making any fuss. These sectors go through a validation process defined by the hard disk drive's firmware. This allows for a reallocation of the problematic sector into the drives secondary defects list (normally referred to as the G-List) and its replacement by a sector from a reserve bank that the drive holds for this very purpose.

Identifying bad sectors for those that are not experienced in recognising the fault can be difficult so watch out for the following:

  • Slow performance, particularly when accessing the hard drive
  • Error messages such as: Data Error (Cyclic Redundancy Check)
  • Chkdsk or Scandisk running at startup
  • Drive failing to be recognised by computer or requesting a format

If you notice or suspect degraded media on your hard disk drive, you have a problem that the drive cannot correct. Many would turn to software solutions to aid in the fight against this problem, however, the drive firmware itself is the best tool to handle this problem, if it can't, don't try to fix it, no software exists, or can exist that can 'repair' a sector that has failed due to physical problems within the drive.

It is extremely important in cases of suspected bad sectors to immediately back up your hard disk data. It is the recommendation of TRC Data Recovery that file system utilities such as Microsoft Scandisk, CHKDSK, or the Unix/Linux FSCK implementations are not ran, and if they are, should only be ran in read only mode (in order to identify the problem whilst preventing data loss).

There is a common misconception that programs such as the above mentioned can 'fix' bad sectors on a hard disk, this is not the case, infact it is often the case that they are quite destructive and can make matters much worse. Many, many files have been non-recoverable due to the running of these utilitites and the file fragment recovery that they attempt.

Our recommendation, having studied many hard disk failure situations resulting from degraded media, is to back up the data on your drive immediately and without hesitation, particularly if the data is important to you! Continued access of the device will lead to stress on the read/write head assembly, which can in turn lead to head failure, or worse a head crash leading to severe loss of data.

Methods of backing up your data in these cases will be looked at in our next article...

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