Filling up external USB drives

All of a sudden, Backup Exec 2010 is completely using all of a 1tb drive with 3-4 backups.

this system used to be able to run weeks without filling up storage.

Nothing has changed and the data being backed up has not grown extraordinarily.

we have been doing a simple GFS rotation with differentials as the daily's.  I can do a full which is about 200gb backup then do 4 differentials and the drive is completly full.

What am i missing?

Thanks for any help!