than most likely it is the setup of your overwrite protectiopn period for the media sets.
Unfortunately - by design (you can not change this), BE first looks in the corresponding media set whether there is a tape with OP time = 0 if yes it takes this tape - if no, it looks for a tape out of the scratch pool (or exactly the other way round, depending on the media overwrite options you have set)
But, if both is not true (no OP =0 tape and no scratch tape ) BE will look in any other media set it finds, whether there is a tape with OP = 0 . If it finds one, it graps it out of that media pool and allocates it to the pool the backup is attached to.
In your case this would mean, you will have to check the overwrite period AND append period for the diff backup pool (effective OP = AP + OP) and of cause the OP for media set A.
If the OP and AP are setup correctly, the easiest way to start is to just load only scatch tapes to the loader. BE will pick the tapes as needed and allocates them to the pools.
You will see, that there is an overwritable tape (or appendable for differentials) within every pool whenever a backup runs. At least it should be.
Hope this helps
Regrads
uz
