disk backup ,volume is full ,but disk not full ?

This is an issue with your media. Check if Networker has marked this media as suspect.

As we are talking about disk, it could be an I/O on disk that caused this or there was something else on that disk that filled it so you could not use file device up to its full capacity. Is that file or advanced file device you are using it? If first one, I would recommend to switch to second one.

As said above, it could be an I/O error. Try to mark the media as Appendable and see if it happens again.
If so, you can try to verify your disks or try advance file type, as said before too.

but the disk ( Includes this volume) Still copy file in

if use adv file , when backup 80GB, networker is remove Unfinished saveset

It will remove aborted savesets and not unfinished (aka in-progress saveset). If something is aborted then it is useless so removing it is preferred thing to happen.