I'm doing a clean install after a crash and have set up Time Machine to backup only Documents and Desktop but the amount TM shows it is ready to backup is 40GB almost exactly twice what the total of the Documents and Desktop folders contain. When I tick "show invisible items" then the folder Volumes appears which won't respond to being excluded from the backup. But if I exclude the Users folder (which makes no sense) then the amount to be backed up looks about right. What am I missing here?