If your CD-ROM drive is also not seen by another (SCSI-savvy) Mac, you may be being forced towards the conclusion that you have a defective CD-ROM drive. If your CD-ROM is seen by another Mac, and if a different external SCSI device is also not seen by the IIsi, the SCSI controller of the IIsi may be at fault. Uncommon, but possible. Another far-from-impossible cause of your problem could be a substandard SCSI system cable between Mac and external drive. If you can, try another cable. SCSI cable goodness is measured by cable diameter. Skinny=inadequate shielding, which leads to confused (parallel) signals in the cable.

de