Hello,
BackupExec will pick up Media from a different Media Set when the Media in the targeted Media Set is not Overwritable or Appendable.
Backup Exec is trying to load media, it is not finding any overwriteable media in the specified media set.
Because of this, it is grabbing the media from other sets and moving it to the media set you are attempting to use.
We suggest you to verify the Overwrite Protection Period and Append Protection Period of the targeted Media Set.
Refer the following Technote which explains about OPP and APP.
http://seer.support.veritas.com/docs/237374.htm
Alternatively refer the following Technote which explains about the Media Rotation and at what stage BackupExec will select a Media from a different Media Set.
http://seer.support.veritas.com/docs/192265.htm
Thanks.
NOTE : If we do not receive your reply within two business days, this post would be marked assumed answered and would be moved to answered questions pool.