>   DL> 512 would be luxury!  Remove 4x16 for the partition table and 2
>     > for the 55, aa at the end, the bootsel setup steals 9x4 (names),
>     > another 55aa, two bytes timeout, flags and default key.  So 512
>     > - 2 - 62 - 2 - 36 - 4 = 404 bytes for the code and messages.
> 
> Who says there are no challenges in software development any more? ;-)

Absolutely, gas doesn't know how to compile most of the addressing modes
either - so you have to write in hex!  It's a good job there are only 400
bytes to play with!
> 
> 
> So, that part after the colon always has to be "netbsd" (e.g. boot
> sd0a:netbsd) ?

No - but it needs to be the name of a netbsd kernel
> 
>   DL> A second system!  I'm testing the mbr a single system.....  
> 
> The second system did not exhibit the same symptoms.  Since it has a
> different (smaller) disk, this may lend some weight to the suggestion
> that it may have been geometry related.

This may be one of those cases where you start wonderering if you were
dreaming the original fault :-)
> 
> I've mostly done what I wanted to with the first ProLinea, so I'll try
> wiping the disk, writing another diagnostic partition, and installing
> NetBSD in an attempt to reproduce its earlier behavior.

Good luck....

    David