When performing scheduled backup, the data currently stored in the DVD disk gets erased/overwritten even if the “Erase disk before Backup” checkbox is unticked.
I think there are problems with the DVD section of Comodo Backup. I have never been able to get it working properly either. Hopefully we will have an updated version coming sometime that will correct this.
Maybe someone else has been able to get this to work successfully. If so I hope they will answer your post.