i wouldn't necessarily suspect the drive.

it's saying that the fworm bitmap claims a
block has already been written, which isn't allowed
on a write-once structure.  thats fine, except
it might have been interrupted at some point when
the fworm bitmap had been written but pointers
in the super block and other structures updating
the allocation addresses had not been written.  the data might
or might not be in those blocks.
actually, i'd suspect it had crashed or rebooted
during a dump.

do the recover again, but try a kernel that uses a larger value in cw.c for
	h->fsize = s->fsize + 100;		/* this must be conservative */

you could also just comment out the fworm diagnostic.
i've done that in the past but i knew it was safe in those
cases because i had a clearer picture of its state when it crashed.