I've just used several utilities to check whether my HDD drive is really failing - I've come to a conclusion that it's the hard disk drive that causes unusual slow downs as my system almost freezes sometimes (HDD light on my laptop is lit all the time when it happens).

It appears that everything is healthy except for "reallocated bad sectors" being at 11. This is the only warning that disk check utilities give me. What does it mean? Is there anything I can do? My drive is running at DMA 5 mode, so that is not a problem.

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    The very existence of reallocated bad sectors means your drive is dying. Bad sectors are normally caused by physical damages to the disks' surface which will normally spread due to the high rotation speed of the disk. You should get a new disk and start migrating your data. – billc.cn Dec 4 '11 at 11:29
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    Hmmm. But is it that bad at the moment? How many bad sectors should alert me that tomorrow I may not turn on my laptop? – Mantas Dec 4 '11 at 12:59
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    1 detectable bad sector is normally bad enough. The exact number your hard-drive can tolerate depends on its model and basically your luck. You should also monitor other SMART metrics like ECC error rate closely. If that goes beyond the threshold, you should replace your drive immediately. – billc.cn Dec 4 '11 at 13:27
  • A reallocated sector from the factory isn't a sign of problems, just a manufacturing flaw. If you lose a sector in use, though, it's time to replace the drive. – Loren Pechtel May 4 '13 at 23:15

If (say) the sectors form part of the catalog B-tree of an HFS Plus file system on OS X, and if replacements are 'distant' from an optimally placed and otherwise contiguous B-tree, then it's possible that:

  • frequent reading across the distances is contributory to slowdowns.

(On a hard disk drive without bad sectors, a contiguous catalog B-tree can have a significant positive effect on performance.)

Defocusing from HFS Plus: comments under the question are valuable. So back up regularly, and consider replacing the drive – before troubles worsen.

Understand that in an edge case, reallocation may cause dataloss …

Suggested reading

The SpinRite (version 5) documentation linked from under an answer (to a Mac-oriented question).

  • @Ramhound I did so. As far as I recall, the more recent documentation did not contain the relevant information. – Graham Perrin Aug 6 '13 at 7:25

Yes, reallocated sectors can cause performance degradation. Consider a sequential read or write operation. If the operation encounters a reallocated sector, the operation is effectively no longer sequential because the read/write head must be repositioned to read the replacement sector (since the original sector was marked bad and the address was reallocated to a spare or "replacement" sector). If frequently-accessed files or filesystem structures are stored in the reallocated sector, then the performance degradation will be even more noticeable.

It appears that everything is healthy except for "reallocated bad sectors" being at 11. This is the only warning that disk check utilities give me. What does it mean? Is there anything I can do?

All the major hard disk manufacturers consider even one reallocated sector reported by the SMART diagnostics to be a serious enough flaw that they will replace your disk under warranty, even if the drive had reallocated sectors straight out of the box. Regardless of whether it's still under warranty, you should consider the drive a ticking time-bomb. Back up your data and replace the drive immediately.

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