I looked at this once and the only difference between VMS and NetBSD is
that VMS sends the START UNIT command to the drives when it probes them.
Drives that are already spinning ignore it and ones that aren't start. 

I stopped when I was trying to figure out if the Monitor PROM's "B" command
actually started the drive before booting from it (I can't believe it
doesn't!) but never confirmed it. I have a SeaGate drive that when I try to
use it to boot NetBSD it just goes away, no NetBSD prompt or anything,
which suggests that the system gets stuck early on in the boot process,
perhaps probing the SCSI bus.

If anyone wants to look into this I would suggest starting with the
standalone boot program.

--Chuck