First - thank you for your suggestion(s). I thought (incorrectly) that I had pursued that avenue in trying to solve the 1801 problem. In fact when I disabled the LAN ports in the advanced/boot menu, the 1801 problem ceased. Hurrah! (I owe you a coffee)
(I have no idea as why this issue should have occurred at this time as the previous BIOS/boot configuration was as such and stable for over two years. To some extent, I don't care, I'm happy to be beyond that...)
But... (there's always that 'but')
I still have the 'operating system not found' problem which is getting to be more than disconcerting. During POST all of the SAS drives appear correctly, going into the cntl-A SCSI setup all RAID sets show properly with the correct set showing as 'bootable', and I ran the (standalone/bootable) advanced diagnostics disc/CD on the HDs without error (all HDs are SAS) and the SAS portion shows no issues with the controller or the physical HDs themselves. As stated earlier, I physically pulled all of the HDs and ServRaid card and reseated them.
It appears that my system (and me too) is confused. Is there some way to inform it that indeed there is a bootable/system on SAS drives 1 & 2 which is RAID set X?
Again, there is nothing(!) that I did that would have gotten near any boot records... (it was a 'normal' power-down / power-up sans the power-up being successful)
I guess the good news is that I'm past the 1801 problem - indeed I still need help with the system not found issue.