If you click on 'System' while in db programming, you will see 'Periodic Backup Save Time' which is the time the db is automatically backed up to the compact flash.

Are you using any CT apps such as Call Center Suite, OAISYS or Xarios? Any of those apps have the ability to manipulate DND.

I'd pull a db save and run it through the tester. If the db is corrupt and it has been saved to the compact flash, the system will continue to attempt to backup from the flash.