If you set the Append and/or Overwrite periods to 0 that means you can't append or overwrite them. If you are writing 1 backup a day and want to be able to overwrite the previous file each day you should set your overwrite period to >1 day (12 hours, 20 hours, whatever).
Are you backing up multiple systems each night to make this 600g, or is it one protected system? Personally, I keep each system separate so that they have their own BKF files, one per day. If it's a full backup of a 200gb system, i will have a 200gb bkf for that one day single backup. It seems to work fine for me on my primary media server that backups up almost all of my servers. I have another issue on another server, but that's going to be its own thread...