It turns out it actually was the enclosure. I overlooked some kind of SCSI extension cable it's got internally: a 4-inch 50-pin male to 50-pin female cable. A couple of the wires on that extension looked kinked, and anyway I didn't need it, so I just removed it. Now the drive works fine on my LC II and my Power Mac 8500, but still isn't recognized by my Plus. My guess is the finicky SCSI in the Plus won't work with that 6 foot SCSI cable, so it looks like I'll still be shopping for a shorter DB-25 to Centronics 50 cable.