Getting back to the original poster's train of thought, this
means that a particular area on your disk is marginal.  The
media is poor, but by re-reading the sector with error correction
the controller can figure out what the original data was.  

I have a disk that is similar in that it has marginal sectors
which various programs on the PC which supposedly map out, but
don't.  It's a handy test disk.

I wrote a script that writes 1M files to fill all available
space on a NetBSD disk and tests them.... sectors with an error
the file is saved (and moved to an out-of-the-way subdir), good
files are deleted, and returned to use.  Bad stuff stays assigned
to out of the way files, which are never accessed.


This works well.  I think it is really the same concept as the 
rabad stuff on old dec systems.

I can post these shell scripts to the pkg dir if there
is interest.

I have also been concerned that NetBSD does an install on a disk
without testing it's integrity....  something to think about.