Stick a screwdriver/small long metal thing into the fan blades at the rear of the motor (make sure youve got enough purchase that theyre unlikely to snap) clamp the body of the gearbox and then try to unscrew the chuck by hand. If that doesnt work get a hammer and tap the edge of the chuck (the un-rotating bit) that you would turn towards if you were doing it by hand - this sudden impact turning force can help loosen the chuck, or at least has on the many drills Ive played around with in my time If this still doesnt work (bearing in mind youve reached the whack the side if the chuck as hard as humanly possible stage) then out comes the angle grinder