it was said:

> > >>>Modern drives deal with bad block substitution
all by themselves.
> > >>
> > >>Umm - not quite, right?  That is, if a block
"goes bad" and you get a
> > >>read error, the drive isn't going to do any
"substituting" at that
> > >>point.  You'll just continue to get the read
error if you try to
> > >>access (read) that block.  It's only when you
allow another *write*
> > >>to that block (e.g. by deleting the original
file and writing new
> > >>files) that the drive will automatically
substitute a spare block for
> > >>the one that went bad.
> > >
> > >
> > >SCSI drives, at least, may do automatic
reallocation on both reads and
> > >writes ( camcontrol mode da0 -m 1, the ARRE and
AWRE flags ).  If the
> > >drive had to reread the block or had to use ECC
to recover data, AND
> > >the entire block was recovered, it will relocate
the data if ARRE is
> > >set.
> > 
> > Good to know, although I stopped buying SCSI disks
(for home use)
> > years ago.  I presumed the more common case these
days, that we
> > were talking about IDE disks.  In fact doesn't
this (from the original
> > question):
> > 
> > ad0s1a: hard error
> > 
> > necessarily refer to an ATA (IDE) disk?  I don't
believe any (current)
> > ATA disks will do automatic reallocation on reads,
will they?  Though
> > of course serial ATA drives seem to be "the
future" and are taking
> > on more and more SCSI-like features as time goes
> Both ATA and SCSI drives may relocate blocks that
were difficult
> to read (e.g. correctable errors, took multiple
attempts, etc).
> But if the block can't be recovered at all, the
drive will still
> report an error to the OS (in addition to


A tool that all may find useful is SpinRite 6.0
available from Gibson Research at It's not open
source or freeware but anybody with an Intel, AMD, or
TiVO system that uses a harddrive ought to have it.
Note: I am in no way affiliated with Gibson Research,
other than having used SpinRite since the days of
manually interleaving MFM drives.



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