Thanks EvilCapitalist and Crutch! As Gorgonops said, I goofed in my original posting. The SCSCI2SD board was positioned so as to allow SD card access through the "expansion port" on the back of the case. As you might imagine, this was placing a fair amount of pressure on the 5-connector harness...
MODERATOR NOTE: The OP requested this be deleted because they believe it may have been a false alarm, they believe the root cause was probably an issue with the wiring harness changing position when they swapped a physical hard drive for an SCSI2SD. This note is being put here instead of...