When my WD20AP unit is attached to an ADAPTEC AHA1542B
under DOS and
allowed to
boot with the drive turned on, it "sees" and reports the presence of a device,
WD-1006-something which it later reports it can't spin up.
OK, pretty common item, a zero footprint, ie it sits under a compact mac
like the Plus, and inside is a bare SCSI drive and power supply, no bridge
thingy, just wires to the SCSI drive from the connectors and ID switch. The
failure mode is about 3:1 drive vs power supply, so many are like yours,
talks but doesn't spin.
If you replace the drive, and the power supply is still good, they are
pretty handy to have around in a mac environment, since almost all macs
support external SCSI devices you can put one of the early flexible OS
releases like 7.5.3 on a small hard drive and then use it to boot test
systems etc. With a larger drive it is handy to both boot old systems, and
to make quick copies of the internal hard drive before formatting etc.