Note that the SCSI bus must have terminators on both ends. If you have
nothing else connected to the hostadapter, one terminator must be on
the hostadapter (look at the BIOS setup, you should be able to switch
the termination on there). The other terminator must be at the end of
the cable. Some cables have fixed terminators mounted at the end, if
yours have not you have to plug in a terminator at the last connector.
Connect the tape drive to one of the remaining connectors of the cable.