If you mean the brake discs themselves, make sure there are no countersunk screws attaching them to the hub flange. If everything is undone give them a good tap with a copperheaded hammer. Sometimes rust builds up between the disc and the hub flange. Once you've got the disc off, use a wire brush to remove the rust and grease the mating surfaces (lightly!) before re-assembly to stop rust building up again.
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