On 11/3/2010 8:32 AM, Keith Roberts wrote:
>
> So to prepare the disk for returning under warranty, I used
> another HDD utility to clean the disk again
...
> So I ran an Advanced r/w scan again with Hitachi DFT, and
> the result was OK.
A complete disk wipe brings bad sectors to the drive's attention,
forcing it to remap them using spare sectors set aside for the purpose.
All drives can do this, and they do it without logging the change. You
can't tell, from the outside, when or whether the drive has done this.
All you can do is infer it, because a sector that once tested bad now
tests good.
As to why this happened only during a format, not during the previous
disk test, it's probably because the format zeroed the disk. That
particular drive may have a policy to only remap sectors on write, so as
to preserve the sector contents for potential recovery later. (See
below for one way this can be done.)
It may be that your drive is now fine.
If you put it back into service, at minimum I would set up smartd, from
the smartmontools package. Maybe run smartctl on it by hand daily or
weekly, too. If you find that errors start happening again, there is
something continually degrading the drive's integrity, so the automatic
sector remapping will eventually run the drive out of spare sectors.
SpinRite (http://spinrite.com/) does nondestructive sector remapping.
At level 4 and above, it reads each sector in and then writes it back
out to the drive. Because remapping is silent, it's possible for it to
appear to do nothing, yet improve data integrity by bringing dodgy
sectors to the drive's attention.
If a sector can't be read without error, SpinRite forces the drive to
ignore the CRC and return the data anyway, retrying many times, then
making a statistical guess about the most likely contents of the sector.
(Reading a bad sector won't necessarily give the same value each try.)
Then on writing the reconstructed data back out, the drive
automatically remaps the sector, repairing it.
You might want to combine the SMART monitoring with periodic SpinRite
runs on the drive until you regain confidence in it.
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