I understand your suggestion, but it is not relevent to my situation because my tapes are large enough to fit multiple daily backup jobs on one daily tape so, for example, i can easily store 4 monday jobs on the monday tape and don't want to erase or...
The problem is that if there is space left on the tape that the job could append to, but not as much space as it needs to backup the entire job, will it know that before it starts and just overwrite, or still try to append in the empty space and fail...