If you can't just pull your brake disc off the hub, after removing the brake calliper, first try tapping the disc from behind with a soft faced mallet. If this doesn't work, there is a threaded hole in the disc, insert an M5 bolt and turn the disc off by tightening the bolt into the hole. make sure the bolt is 10mm longer than the thickness of the disc.
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Any work you do to your car is entirely your own responsibility. If in doubt, you should check technical advice with an independent , qualified person who has seen your car. The MX-5 Owners Club, its officers and forum contributors accept no reponsibility for any damage caused to your person or property as a result of you following or not following the advice offered on this forum.