hi,
as i understood from above posts , you are having some pool assigned and media is there in this pool but when you are starting the backup it is not taking media from that particular slot , it is taking from other empty slot.. if i am correct then please do the following.
. recycle the media.
export the media
and import the media again into same pool.
start backup as usual it will run hopefully.
best of luck.
regards:
Bhupendra