If your internal HDD has a SCSI ID=0 (usually achieved by having no jumpers on the pin pairs A0, A1 and A2 in the header block found either on the component side of the drive's logic board, or on its logic board at the front of the drive), it leaves SCSI IDs 1 and 2 for further HDDs. Apple conventionally (from about the Mac II series onwards) assigned SCSI ID=3 to optical drives. The ID is set by jumpering pin pairs at the rear of the optical drive. If no-one has fiddled with the drive before you got to it, it should have left the factory with jumpers already in place on the pin pairs labelled TERM POWER, 1 (meaning A1), 0 (meaning A0) and PARITY. Restore those settings if they are not present, and fit a CN-50M terminator to the unused CN-50F member of the pair of sockets on the rear of the case.

Enabling supply of (SCSI) termination power on the optical drive is unlikely to do any harm, and is absolutely necessary if the IIsi has insufficient termination power available, or if you have a very long SCSI cable from Mac to external CD-ROM. Either way, the physically last device in a SCSI daisy-chain must always be terminated to dissipate electronic hash in the daisy-chain. Termination can be purely resistive or achieved with dedicated ICs. It should not be confused with termination power, which provides the bias/power to make a terminator work.

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