I just setup about six LC3's and ran into the same problem on two of them. I have
no idea as to what the problem is, but here is how I triggered it, and my
solution...

I was using the same MAC OS7.5 hard disc to perform the installations on a number
of hard discs (I love SCSI!!). The Mac partition was SCSI ID0, and the target
(NetBSD) drives were SCSI 1 and up. After a complete install, I had it in my mind
to place a hard drive with the fresh NetBSD install in a LC3 and re-enter the
install program to perform a "fstab force" before the initial boot. This was
required to re-sync /etc/fstab to the correct device now that the NetBSD root
partition was "/dev/sda4". For some reason, the initial boot corrupts the root
partition when /etc/fstab is out of sync. I forgot to do this on the two "bad"
LC3 installs.

My solution: ensure to re-run the installer and execute "fstab force" after
re-arranging any SCSI drives.

hope I wasn't too confusing (It's past my bed time).

brian