My time machine has decided it doesn't want to backup anymore, i have a 1TB external hard drive everything was working fine then it "failed to backup not enough space" but there is 144GB left, also i can't go back to any files beyond the last backup, i have run first aid and no problems, ejected and restarted still won't backup, this is about as far as i'm confident of going, is it worth reformatting and starting again. Read so many solutions but they seem beyond me just need simple help.