You need to load ide-scsi module. Try to enable backup via server-manager and restart your server. It should be done automaticaly.
If not, verify that in the /etc/rc.d/rc.local file where is line at the end like this:
/sbin/modprobe ide-scsi > /dev/null 2>&1
This will enable your IDE tape drive to emulate SCSI drive /dev/nst0 or /dev/st0 where "st" is for "SCSI tape"