With a RAID1 (Mirror), if one disk fails the other should run the array in degraded mode. As there was no critical data on the array I was in the process of testing various scenarios, one of which was a critical absolute failure of a disk, simulated by booting the machine without the drive.

That's when I ran into OpenE not booting with the "Unknown Operand" error, and have so far failed to both resolve it, and figure out what's causing it. The testing continues.