Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 106

Desktop computers support up to four drives. They may be
installed in various configurations, as shown and described in
Chapter 1, "Your Computer at a Glance."
When installing additional drives, follow these guidelines:
For optimal performance, connect hard drives to the primary
controller. Connect expansion devices, such as CD-ROM,
IDE tape, and diskette drives to the secondary controller.
You may install either a third-height or a half-height drive
into a half-height bay.
You must install one guide screw to ensure the drive will
line up correctly in the drive cage. Compaq has provided
extra guide screws, installed in the front of the computer
chassis, under the front bezel. Some options use M3 metric
thread hardware. The Compaq-supplied metric screws are
black.
Optional drives are available from Compaq in kits that include a
special drive cable. The configuration of the drives employs a
cable-select feature that identifies the drives as device 0
(primary drive) or device 1 (secondary drive). The system board
determines which drive is device 0 or device 1, based on the
way the drives are connected to the special drive cable. The
device 0 drive is the drive connected to the short segment of the
drive cable (or that connector closest to the system board); the
device 1 drive is the drive connected to the long segment of the
drive cable. The device 0 and device 1 identities are used by the
system to distinguish between the two drives.
The installation requires no jumper setting changes on the
existing or optional drives. All Compaq drives have the jumpers
preset for cable-select installation.
If installing a second device on the primary controller,
you must use an 80-conductor Ultra ATA cable for
optimal performance. This cable is available as a Compaq
option.