The SSL 3.5" drive is a TEAC FD-235HF IDE drive, with a TEAC FC-1 SCSI to IDE controller board. One required per drive, and set to drive numbers 0 and 1 IIRC.
Had plenty of drives die in the past. They are not common, and very specifically configured for this application. I have a contact who has a lot of NOS SSL drives, but would suggest it may be the Bus Driver card in the computer, which I have found to be the first thing to fail and prevent a boot sequence.
Anthony.