An interlock system is provided for removable-media drive unit to prevent the drive loader from attempting to pull in and mount a data cartridge before it is released by a mechanical gripper or human hand. The interlock system includes a light source and a photodetector positioned in front of the drive on opposite sides of the path cartridge takes when it is being inserted or removed from the drive loader. The presence of the cartridge, gripper or other object in the path interrupts a light beam from the light source thereby signalling the drive controller to disable the drive loader. When the gripper has released the cartridge and withdrawn, or when the path is otherwise clear, the photodetector again detects the light beam and signals the drive controller that the loader can be safely enabled. In one embodiment, the light source and photodetector are mounted on a card which plugs into the face of the drive. The presence of the card indicates to the drive controller that the drive is installed in a library environment and causes the drive controller to execute appropriate program code to interpret signals from the photodetector and enables and disables the drive loader in response.