Can Ghost 15 automatically optimize storage seperately on different drives? Doesnt seem to

We have Norton Ghost 15 running for daily backups.  Each day runs a backup to a different ext drive.  Each ext drive is a seperate backup destination in Ghost.  They are all set in "Manage backup destinations" to only use 80% of the drive and only one drive runs per night.  Even though it seems Ghost 15 has the ability to manage different drives seperately, it seems to be getting confused.  For example, drive B starts to approach 80% full and tries to delete a backup that exists on drive A.  We don't want it to do this, we want each destination to have it's own list and library of files.  

We read through the documentation (all of it) for Ghost 15 and it's not really clear on this subject, although it seems as if it's setup to do what we want (manage each drive and automatically delete points as needed from each drive INDIVIDUALLY).

Anyone use this feature with multiple ext drives and Ghost 15?  What are your results?

Thanks