I now have the tape drive working on my server and I was able to make a successful backup. Unfortunately it introduced a new problem: I can no longer burn CDs.
The tape drive (a Seagate TapeStor 20) is connected as the slave device on the same IDE controller with the CDR/DVD combo drive. Both use the ide-scsi driver which makes them look like SCSI devices. When the system boots both devices are recognized and identified as SCSI devices, but cdrecord isn’t able to access the drive & scanning the bus fails to identify it.