What happens when my readynas backup drive gets full?

I have a 4tb hd hooked up to one of the Nas's USB port.

I've seen a few atricles that talk about rsync but I can't seem to find that as an option anywhere on my menus.  Maybe that's just for x86 Readynas systems(?)  

The backup options I have set for it to do a full backup just the first time.  So, apparently it's been doing incrementals for a while.  However, I can see the backup disk is getting full, hence the question with the obvious follow up of,"If it doesn't handle it automatically, what should I do?".  I see there's an option to remove the contents before a full backup but since its set only to do a fullbackup the first time, I would think that's a non-starter.  What I don't see is an option that says, "If it fails an incremental, switch to full and remove" (the latter might require specifying the remove but so be it).

Anyway, since I don't see such an option, that would suggests that the way it would work with my current options is somehting like, it tries to do an incremental, it fails due to lack of space, it, I hope as I have configured my readynas, it sends me a backup failure message.  So then I would manually reset the backup by telling it to to do a full backup with the aforementioned remove contents box checked and then, aftter it completes that, manually go and switch it back to only due a full backup the first time.   This is way to manual.

Another  (available) option seems to be to set the backup to do a full backup every so many weeks with the remove contents before a full backup option checked.  That should be fire an forget.   I guess I would have to, a priori, estimate how many incrementals it might take for my disk to be full and set the bull-backup weeks number accordingly (w/r to my backup shedule which is daily).  Obviously, I'd want to be conservative so as to not miss any incrementals due to the disk being full but the schedule not ready to do a full backup.

Do I have this right?