G drive SSD for back up.

I have a 4 tb G drive SSD. I was trying to back up my Mac pro for the first time as another drive has corrupted.

So when I look at the G drive there are 2 volumes, Utilities and G drive Mobile SSD. The latter has stuff on it and Time machine won't back up unless it erases it all. No thanks. G utilities is empty atm. SO I said to time machine to use the utilities bit. When going through that menu Time machine asks if you want to limit the amount for back ups do I did to 1.2 Tb ( I think) . But then when the back ump runs it says there isn't enough room. Maybe I said gigabytes or something by accident. But how do I reset it as I can't find a way to do it. Surely it's not permanent. I want my space back