>I grabbed a copy of the diskimage to install on an "empty" pmax.  After
>hanging the disk off an old 1.2G installation, I do the following:
>
>        disklabel -W /dev/rrz1c
>
>Which seems to work.  When I try to dd the diskimage by 
>
>        gunzip -c diskimage.gz | dd of=/dev/rrz1c bs=10240
>
>dd fails with something like 
>
>        /dev/rrz1c: read-only file system
>
>or something along those lines.
>
>
>Disk in question is a previously labeled rz58.  Am I missing something
>brutally obvious?

It depends how old your 1.2G kernel is. If it's really old, the
disklabel gets marked unwritable as soon as the open-count for that
parititon goes back to zero.  This is a (mis)Feature in the disk
driver that dates back to 4.4BSD.

The standard work-around is (from a shell with job control):
	dd if=/dev/rrz1c of=/dev/null bs=10240 count=10240

	<suspend, stopping the dd in progress with /dev/rrz1c open>

	then do the disklabel -W and dd.