This message was posted on behalf of Intel Corporation
I recommend you to take the serial number of the HDD you used to have connected, then plug it again to your computer and disable it for the RAID configuration.
Hope this help.
Thank you very much for your response. I have progress to report.
It turns out, around or exactly when the array rebuild completed, the 'ghost' drive vanished. I suppose it could just have been kept around until then for some reason. This seems like a thing that should be noted in manuals. Or perhaps I should have just known.
I do appreciate having received a response though.