How to prevent USB drive being added to backup?

Hi,

I am running BE 2012 to back up all my servers.

There has been a problem with backups taking a vast time to run over the last two days.

It appears that the backup job that was supposed to backup a single selected drive also added a large USB drive to its selection that I had temporarily connected to the machine.

How do I prevent BE2012 automatically adding USB drives to the backup selection in this manner?

Thanks,

Julian