Do you really want to remove all files? If so, just reformat the drive and see what it's capacity shows after that.
I tried to remove all files from my 30GB USB drive.
Then it reported 8+/- GB still in use.
I checked Show Hidden Files, Show System Files, etc., but nothing showed in Explorer. Finally I tried Disk cleanup. I showed 0 to be gained by emptying the recycle bin; yet when I chose View files, I found about 3 GB lurking in there...deleted them. Now 5 GB hiding somewhere. I tried DskChk and found 3GB of fragments. Now 2GB and out of ideas.
How can I find those files?. Does the USB drive need any stored files to function?