I think writing the question in a post puts my thoughts in order as I sometimes find the answer shortly after.
A BIOS setting seems to fix the issue. Checking the HARD DISK CONFIGURATION under the BOOT tab in the BIOS with just the two RAID drives attached shows...
Connecting the 3rd drive and cycling power the BIOS then shows...
So disabling the SS one from the boot configuration and restarting. System booted up ok a few times in a row with the 3rd drive mounted in /media/backup.
Maybe this will assist someone else with this issue.
Now on to the 1u chassis overheating the CPU with the cover on. Ugh.