Easy...... The drum is corroded in place. Either it's rear surface or......more likely...... Where it is a tight fit on the hub. After you have removed the securing screw, run a couple of drops of oil or WD40 onto where the drum sits on the hub then...... You will notice there are two small threaded holes on the face of the drum. (From memory, about M6) If you screw in two bolts or screws into these holes, they will pull the drum off (or at least un-seize it). Take it slow, a little on one side then a little on the other. That's what the two small threaded holes are for. Ian.