I'm not real familiar with the disc brake rears, there aren't possibly any countersunk screws securing the rotor to the axle??? Some of the Metric rears with drum brakes had little spring washers screwed on the threads to hold the rotor in place. If not, try a little penetrating oil on the rotor where it seats on the axle end, they have a tendency to build corrosion in there. When all else fails, try a torch with a heating tip and a low flame. Heat the rotor at the point where it attaches to the axle....