I talked to BT tech support and with my own troubleshooting it seems I had 2 or more of the drives fail/corrupted. Originally BT tech support diagnosed a bad logic board in the unit but my advanced replacement unit had the same issue when I transferred my HD’s into the replacement unit. BT sent another unit and I had the same problem, again.
I put the HD’s, sent with the replacement unit, into my old box and “voila” the unit is recognized in Windows Explorer and working. Over 800GB of music & 140GB of photos/vids gone ... sigh.
I guess I’ll be buying some more HD’s to back up this unit.
Ah, the joys of computing and the tenuous shelf life of data.