As Softrak Support suspects, those files are are usually left around when a backup does not complete properly.

You should run your DC job manually and check that the backup file created is ok (do a test restore of the file to a temporary area to be sure).

You might want to review the backup files in the folder and remove the .z99 files and any others that don't appear to be complete. Monitor over the next few backups to ensure that all is well. You should not be getting the .z99 files.

Pete