Compaq EN Series Hardware Reference Manual page 31

6. Install two guide screws on each side of the adapter 1.
7. Install the adapter in the desired drive bay 2 by sliding it into the
front of the drive cage; the drivelock automatically secures the
adapter when it snaps into place.
The primary 3.5-inch diskette drive should only be installed
into bay 3. Bay 3 is the bottom bay in the minitower
configuration and the topmost bay in the desktop
configuration.
Aligning the Guide Screws and Installing an Adapter with a 3.5-Inch Drive
Hardware Reference Guide 2-23