IBM RT PC Options Installation page 70

3. Hold the drive with the printed circuit board on the bottom and the
connectors toward the center of the system unit. Place the drive on the
side rails and slide it into drive position E.
Push the drive in until the plastic latches snap into position, holding the
drive in place.
Third Fixed-Disk Drive
In Position E
Section 2. Removing and Installing Drives in IBM 6150 2-31