[ILUG] [OT] reason for replacing a HDD....

[Jakma, Paul]

> However if you are looking for a way to recover from a bad 
> sector I think
> the only way is a low level format utility. I believe some 
> companies (WD
> springs to mind) provide low level format utils which hide 
> bad sectors from
> the user so that when you format the drive it appears undamaged.

explantation i heard was that with time the drive heads become slightly
misaligned with the tracks on disk. A low level format rewrites the tracks,
and thus the drive head and tracks are in alignment again. The bad blocks
are literally gone.

if problem was just confined to alignment, then probably a low-level format
will give the drive a new lease of life. if it was due to something else, eg
damaged media, the bad blocks will come back quick.

> As far as I know HDD manufacturers use this technique when 
> they don't get
> 100% good sectors in their manufacturing process. In fact in 
> the "good old
> days" when processes weren't so good I heard that HDD's used 
> to come with a
> list of sectors which were bad.

that's scsi. SCSI drives maintain several lists of bad blocks. there's the
manufactured bad block list, and the grown bad block list. The manufactured
list always has some entries. The grown list is the one to watch. 

(i think, iirc) ide drives, in the name of cheapness, just have a number of
reserved blocks invisible to the OS. The disk transparently replaces bad
blocks it finds with reserved blocks. when the reserved blocks run out, you
start seeing errors. so when you start getting bad block errors on an ide
disk, it means there's already a whole bunch you don't know about.