Private communication with Arne H. Juul has shown that the most likely 
reason why my DS5000/120 doesn't boot is that the Ultrix disklabel has 
not been removed successfully.

In fact, when I try (on Arne's suggestion)
# dd if=/dev/zero of=/dev/rrz2c bs=512 count=32
it replies
dd: /dev/rrz2c: Read-only file system
and doesn't put anything out. Writing to /dev/rz2c instead succeeds, but 
does not seem to wipe out the Ultrix label...

I conclude there is some mechanism in the NetBSD-pmax disk drivers or so to 
check for an existing Ultrix label and set the disk read-only if one is 
found. How can this be circumvented? How can I get rid of the Ultrix disk 
label?

Helge