Compaq Armada M300 Series Reference Manual page 61

The primary hard drive cannot be used in the mobile
expansion unit MultiBay. For information on using hard drives in
the mobile expansion unit MultiBay, refer to Chapter 6.
1. If the computer is on, save your work and shut down the
computer.
To ensure that the computer is not in hibernation, turn the
computer on, then shut it down.
2. Turn the computer bottom side up, and remove the hard drive
screw .