Excerpts from netbsd: 8-Jan-98 Boot failure after upgrade .. Andreas
Gustafsson@arane (1358)

> The sysinst upgrade procedure upgraded the boot blocks at the
> beginning of the NetBSD partition, but it did not touch the MBR.

That's how it should work.

> I believe my MBR actually contained a NetBSD stage 1 boot block,
> possibly from NetBSD 1.0 or 1.1 (I can supply a copy of the MBR in
> case some MBR taxonomist would like to try to identify it).
> This MBR was able to boot NetBSD 1.2 just fine, but was apparently
> incompatible with 1.3.

The first sector of the NetBSD boot is not a standalone program,
it is simply the beginning of the boot code. No wonder that the
boot fails if the pieces don't fit together.

> Was my setup somehow strange, uncommon, or even incorrect, or has
> anyone else run into the same problem?

I'd call it strange. Normally, the MBR is supposed to contain a
generic piece of code.
If you don't want to keep OS-BS, you can install such a thing by
either DOS "fdisk/mbr" or NetBSD "fdisk -u" (iirc).
If you want to experiment, you could also try to recreate your
previous setup by "dd if=/dev/rwd0a of=/dev/rwd0d count=1" -
you'll have to set up the partition table freshly after that.
(But I can't recommend it, and it might happen that it doesn't
work in the future.)

best regards
Matthias