Simon is absolutely correct.

the only niggle is that if you want to try multiple disks on one
system, you odn't have to setenv boot each time. You can do

    boot -f  rz(0,2,0)/netbsd

at least on the PROM versions i've seen.

Hm.  I know I wrote a manpage that had these details.  Simon: if
people are getting machines without hardware guides, maybe that text,
or some reworked version, should go on the Web page?

Last, somewhere, (if I can find the docs), there's a PROM test command
that should be able to do a destructive read/write test to a disk.
(try "test ?" for a list).  If all else fails, you can try that on the
3gig disk.  

But it does sound awfully like cabling. Is the 3gig disk a wide disk,
by any chance?  The external connector on the 3100 is sorta the same
as a wide SCSI connector but the wrong gender. The cabling is NOT
compatible with wide SCSI, and usually goes to a Centronics-style
connector.  Are the connectors within spec?

If you have internal cabling in the box, is it within spec and in good
condition?  We've had 2 or 3 cases here where internal cabling was
marginal, and where the Ultrix drivers would retry and cope OK, but
the NetBSD drivers were more finicky.  Replacing the internal cabling
sovled the problem.  (ok, 2 of them were on a 3min, but its worth a try).