Open Backup Exec button -> Configuration and Setting -> Audit Log
I found several message about "OSTXXXXXXX has been overwritted" happened when backup job was started to run. Please refer to attachement
OST media are kind of "special". They are kind of like B2D images and kind of like tapes. Basically, OST media/images/files are single use. Internally, they cannot be appended to or overwritten. In place of that, when BE wants to re-use an OST media, it retires it, deletes it and recreates a replacement with the name name/number. To the user, it simply appears that the media was overwritten. The retiring & deleting get logged in the audit log, sample below.
4/4/2013 1:43:09 AM server8\Administrator Device and Media Media : OST00000392-419321DDBA43EE36 has been moved from media set Internal Disk Images to media set Retired Media.
4/4/2013 1:43:09 AM server8\Administrator Device and Media Media : OST00000392-419321DDBA43EE36 has been deleted.
4/4/2013 4:30:26 AM Device and Media Media : OST00000392-419321DDBA43EE36 has been overwritten by job OST1 Backup 00006-Full.