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
The SCSI cable.
As SCSI cards are a somewhat old technology it is very hard to acquire them trough brick and mortar chain stores, although some of the local retailers in your community may still have a few around from back in the day.