If you turn the optical drive cage upside down, you will see two tenons,
which should be inserted into the mortises on top of the hard disk drive
cage when you replace the optical drive cage on top of the HDD drive cage.
Mortise
Tenon
1.6.2 Remove Hard Drive
The HDD drive cage is fastened to the chassis with one screw. Remove the
screw, and push the HDD drive cage toward the rear of the chassis until it is
detached from the chassis.