Too bad I didn't spent a little more time investigating this issue
when I ran into it myself a while back. I just confirmed that after
using "disklabel -W" on the raw disk device then you can then dd to
the entire disk without encountering the "Read-only file system"
error.
# dd if=/dev/rra1a of=rx33.dsk bs=512
ra1: RX33 size=2400
2400+0 records in
2400+0 records out
# ls -l
total 1200
-rw-r----- 1 root 1228800 Jun 8 22:56 rx33.dsk
# dd if=rx33.dsk of=/dev/rra1a bs=512
write: Read-only file system
2+0 records in
2+0 records out
# disklabel -W /dev/rra1a
# dd if=rx33.dsk of=/dev/rra1a bs=512
2400+0 records in
2400+0 records out