Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 144

All SCSI hard drives must be either internal or external, but
never both. However, the system accommodates a
combination of other internal and external SCSI devices,
such as SCSI tape and CD-ROM drives.
When an IDE hard drive and SCSI hard drive are both
installed, the system can boot only from the IDE hard
drive.
Before you can install a SCSI device, you must first
verify the SCSI ID of the drive and, if necessary, set the
SCSI ID to a new number. Be sure that each SCSI device
on the SCSI chain has a unique SCSI ID number.
Second, you must determine if the device ought to have
termination enabled or disabled. Set the termination if
necessary.
Depending on the model, your computer may include one of the
SCSI cables shown in the next illustration.