there should be a little play in it, but a inch seams like alot. the rotor should float a little from left to right and some front to back, so it stays in track the same way a floating secondary clucth works.
if you take off your chain case and gears you can pull off the rotor there is a little key that slide into a notch on the jack shaft and a notch on the rotor maby that has gotten worn, but not likely. I would just try to learn how to ride with out a brake, there for slow people