You may want to read up about the difference between Volume Group and Volume Pool.
Volume group is associated with location - in or out of the robot.
Volume group will change when media is moved out of the robot or into the robot.
Volume pool is used for logical groupings for backup purposes.
Volume pool will stay the same - whether tape is in our out of the robot.
Unexpired media (containing valid backups) cannot be moved to another pool.
See this TN: