The final SCSI device in the chain of devices must be terminated. If you have only one device and it's attached to a SCSI port, you must terminate that device. If you have two or more SCSI devices that are linked together and to a shared SCSI Port ie: SCSI Port, Device #1, Device #2, etc, then the final device in the 'chain' must be terminated to allow for proper recognition. The SCSI Port also requires a terminator as well.
SCSI chain
Identification
terminanting resistor
SCSI uses a parellel communication chain and SAS uses serial Point to Point communication
Simply terminate each of the devices. Terminate the hard disk as well as the external cr-rom.
SCSI
SCSI-2
DB25 SCSI connector50 pin SCSI connector
SCSI 3
SCSI, Fast SCSI, Ultra SCSI, Ultra2 SCSI: 8 devices Wide SCSI, Fast Wide SCSI, Ultra Wide SCSI, Wide Ultra2 SCSI, Ultra3 SCSI, Ultra160 and 160+ and Ultra320 : 16 devices Of course, the number of devices supported depends not only on protocol limitations, but cable length, number of cables used, etc.
The SCSI cable.