Automated Backups fill USB Hard Drive

I am trying to use Ghost to do automated backups with a USB flash drive.  The Flash drive (64mb) is big enough for 3 backups.  When I try to do an automated backup for the fourth time, I get the error message: Error ED800016: The target disk is full. (UMI:V-281-3215-607

 

I have clicked the button that says: "Automatically optimize storage."  It still doesn't deleted the old backups.  How can I fix this?