2 ideas/things for you :
1) I also use a magstar but this is a magazine one, that means i can insert 10 tapes in the drive and the system automatically skip to the next one when one is full. The only requirement for this is THAT ALL THE TAPES MUST BE INITIALIZED BEFORE USE, because the system assume the next volume is the same label.
2) What’s the command you initiated for this full backup : Point 21 of Menu save ? What’s the message ID you get and at which step ?
3) Also are you just using the OS level backup tools or BRMS? If you are using BRMS you can establish the tapes and that when you back up it cleans them before use.
4) It should not be giving you an error at the end of the tape, that is not normal. Can you also give the CPF message when you update the question please.