Explanation of Media - How to resuse a tape asigned to a MP

Hi,

Exuse my newbie question, but I come from a BeckupExec background where things are much different...how can I reuse a tape that has at least one image on it and is assigned to a media pool other than Scratch? I tried to expire the media and then move it to the Scratch media pool, but NB wouldn;t let me saying the tape is already assigned or has images on it.

Please explain...in BE all I had to do was drag the tape from the media pool to the Scratch pool and done!

Thanks