Yamaha cd-r scsi drive disappears after

I have a Yamaha 4416 SCSI cd-rw drive connected to an Adaptec 2930 SCSI controller along with external scsi zip and jaz drives). When I boot up after newly installing the cd-rw, it is detected and functions correctly. After leaving the machine on for a day or two, the cd-r disappears from the machine. After I turn it off and disconnect the cable and reconnect it, the cd-r returns to the machine for another day or so and then disappears again. Termination for all devices is correct. The cables have been changed three times. The CD-RW has been changed once. The same problem has been happening with an Adaptec 2910 SCSI controller that was in the machine first. I am very bewildered.