My SSD keeps running out of space. No matter what I clear it's soon at sub 10 gig.

Mist apps/files etc are on other drives.

I am getting notifications from Support Assist that there is not enough space to back up system files, I have not consciously asked it to back anything up. The implication is that it is backing files up, or at least attempting to. 

Could this be the source of my problem? Are there unwanted back ups clogging up my SSD? Can I find out and if so how would I remove these files and cancel back up. I see no relevant setting in Support Assist for back up.

Thanks

Alan