I had the two main ROMs mixed up ... sorry!
The error displayed is (of course
) not in the Owner's manual. A "real" Rainbow has a strip of LEDs for diagnostics ... maybe that could be emulated as well?
The disk drives are DEC RX50's,
e.g. DEC RX50
I found a Rainbow 100 diagnostic disk in a stack that I had not imaged until now ... maybe that helps once a disk drive is added. DEC Rainbow 100 Diagnostic Disk