Apple Workgroup Server 9150/120 - Manual (Page 63)

5 Make sure that only the last device in the SCSI chain has a terminator.
To ensure accurate transmission of information, a terminator must be at each
end of a SCSI chain. Your server, which is the first device in the external SCSI
chain, has a built-in terminator.
Some external SCSI devices from manufacturers other than Apple have a built-in
terminator. (Check the information that came with the device.) If the device at
the end of the SCSI chain does not have a built-in terminator, you need to
attach an external terminator.
Have your Apple-authorized service provider remove any extra built-in terminators.
You can attach or remove external terminators yourself.
Note: If only one external device has a built-in terminator, rearrange the SCSI
chain so that device is at the end.
IMPORTANT Always turn on any external SCSI devices connected to your
server before turning on the server itself. Otherwise, your server cannot
recognize the SCSI devices.
External SCSI terminator
Connecting hard disks and other SCSI devices 47