I've generally been able to backup to floppies that have a previous version of an UFT backup, with only an initial warning that I am overwriting a previous backup. I generally try to keep a set of backup disks together but I've been able to replace a disk with a fresh blank one (e.g. when the original 6th disk was damaged) with the program not being upset.
I've also mistakenly backed up to disks that were not empty of other files, and just ended up with more backup disks than I wanted. But it did work.
If you get a "disk full" message on your first disk (or possibly others) it probably means that your computer thinks there is not much room on it to even start a backup set of disks. If I have any problem with a disk, I discard the disk, since it is not trustworthy enough to be used as my UFT backup. (I may keep it for other, less important, tasks.)