Nick Holland wrote:
quoted text
>> And in the end, a 
>> '0'-sized bsd.sp after moving in a healthy bsd.mp.
>> I would not totally exclude an interference of this (new?) code that 
>> lead to the described situation. Honestly, nothing at all done in that 
>> session aside from what I wrote, between the 2 boots. I guess, nothing 
>> of what I did should hurt the /boot?
>>     
>
> well, something did damage /boot.  I doubt it was anything you did 
> intentionally, but something also caused a bad bsd.sp to be copied over. 
>   Possibly related.  May indicate system problems of some type.
>   

Okay, back. Works!

But we should not stop here.
Because at mounting my / on /mnt, I noticed that the /boot had also 
taken to a zero size. Like that bsd.sp, which was okay, but received 0 
after copying bsd.mp to bsd. What would now make /boot zero?

/usr/mdec/installboot -v boot /usr/mdec/biosboot sd0
says something like
boot: boot proto: /usr/mdec/biosboot device:/dev/rsd0c
no error message, but /boot is still '0'.
Then I removed /boot, and then an error message came up:
... boot: No such file or directory
Meaning, it couldn't rectify the /boot of size 0.
Last chance: I copied /usr/mdec/boot to /mnt/
Again:
/usr/mdec/installboot -v boot /usr/mdec/biosboot sd0
boot: boot proto: /usr/mdec/biosboot device:/dev/rsd0c
boot is 3 blocks x 16384 bytes
fs block shift 2; offset 63; inode block 24, offset 936
using MBR partition 3: type 0xA6 offset 63
Now, this looked promising and actually worked.
I still take a bet on a round of drinks that there is a bug in the 
recent install/upgrade code that has a tendency to render files to zero 
size.

Thanks for all the input to get this production box back!

Uwe