[OAC_Sparky]

The problem usually is rust that forms between the center hub and rotor. Wire brush really well around the center hub -- the rust expands and keeps the rotor from sliding off. Spray a little liquid wrench (penetrating oil) in the stud holes. and around the center hub, let it sit for an hour. Then use a pair of hammers hitting on opposite sides to "rock the rotor off".

When you re-assemble use a thin coat of silver anti-seize between the hub and the backing plate (the part the lugs are attached to) and the rotor so next time you don't have the same problem. Just keep the stuff away from the braking surfaces.