Backup Exec 2014 adds USB drive to backup selection automatically

I am trying to run a daily backup (same selection list) to a storage pool of usb drives each configured as disk storage.  There are 5 disks setup and are all part of the storage pool.

The job works fine apart from Backup Exec automatically edits the selection list and adds the usb drive to the job when running?  So in effect, the backup job runs, backs up the required files but also the disk itself.  Why does Backup Exec see it right to add the disk to a job I have created? I don't want it to see the disk and think, I will back this up even though the user hasn't specified it in the backup.

Is this just a default behavior I can turn off?

All help welcomly received

Thanks