Backup sets are not overwritten on USB or SATA drives which are swapped on daily basis when Disk Storage runs out of Disk Space. Additionally NAS connected drives that periodically go offline may experience the same condition.
A hotfix is now available for this issue in the current version(s) of the product(s) mentioned in this article. Refer to the Hotfix link under Related Documents at the end of this article to obtain the hotfix needed to resolve the issue.
Backup Exec 2012 Rev 1798 Service Pack 2 (TECH203155)