Excerpts from netbsd: 2-Feb-98 AOpen mb won't boot NetBSD Gunnar
Helliesen@bitcon. (3217*)

> As long as there is (or has been) NetBSD bootblocks on _any_ HD
> connected to _either_ IDE controller on either an AP58 or AP5T
> motherboard it won't boot.

Do your NetBSD partitions start at the beginning of the disk?
I have 2 boxes which hand in the BIOS HDD probe if the primary
bootsector is the NetBSD one. Leaving 1 track free and
installing a generic MBR or os-bs helped.
I can only guess about the reason, perhaps it tries to care about
partitioning and falls into a recursion if a partition starts at (0,0,1).
(It's a "Power BIOS" by either Award or AMI.)

best regards
Matthias