I managed to smash my /boot blocks on both my disk, and my backup disk.

recovery was fraught. 

I was reduced to downloading a
 NetBSD 2.0 .iso image, using it to get first-level bootstrap, interrupt,
set dev=hd0a: (interesting that the boot level thinks in terms of hd<x>)
and then re-re-re-re-re-re run combinations of the above.

fdisk -u -c /usr/mdec/mbr_bootsel

followed by

installboot -v -m i386 -t ffs \
	-o timeout=5 /dev/rwd0a /usr/mdec/bootxx_ffsv1 /boot

did *NOT* work about 10-20 times. I even tried older /boot values off older
releases. 

I still don't know what brought it back. I think wiping the wd0d (ie sector
0) state was important.

The threads in NetBSD email are also moot. Steve Bellovin has seen this at
some point in the past, there is discussion as recently as 2004 Dec about
it, but it seems like the position is 'SOL' or 're-install' with a possible
dd to the first 100 blocks on the disk.

Kinda shame that none of the self-hosted tools can reliably bring back a
lost 8192 bytes  or less of booting linkage. 

-George