> (i'm
> afraid i'm not sure of what fsbn/bn/cn/tn/sn mean).

I'm not sure, but this sounds right:

fsbn = file system block number (block reference number assigned by file
system)
bn = block number (system block number from raw block device?)
cn = cylinder number
tn = track number
sn = sector number

cylinder/track/sector specify the exact place on the disk where the error is
occurring.  Not sure if there's anything you can do with this under NetBSD
(and I can't check, because my hard drive went south on me on Friday -- hope
this isn't a trend!), but it may help if you get some low-level diagnostic
software from the manufacturer.  Back in the day (or just before -- this
would have been ca. 1985) I had a machine that came with a
bad-sector-mapping utility, that would allow you to specify a bad sector to
be replaced.  I think modern drives do this automatically now, but it might
be worth a check to see if such a utility exists.  Or there might be some
option to specify a bad block when you invoke mkfs (as I said, I can't
check!).

Good luck!
-- Tracy Nelson