Might need to apply a little penetrating oil/PB blaster at the lug nut studs to loosen them up and then give the rotor edge a good whack...you're not gonna reuse it anyway so don't need to be gentle...
the small hole is to be able to adjust the parking brake shoe if needed, this is sometimes covered up with a small rubber plug,
Taken from the How To & FAQ section on changing your brakes (may want to check it out, Ilya put a lot of work into that one, plus some good comments/tips in general for the M:
2) The small rubber plug in the rear rotors covers the parking brake adjuster. If you place the car in neutral and rotate the rotor to the 12 o'clock position, remove the plug, you will see a cog-wheel on the adjuster, adjusting this all the way down will "bring in" the shoes of the parking brake on the inside of the rear hub. These can tend to stick and prevent the rotor from coming loose. Closing the adjuster will let the rotor move freely.
so, based on the diagram, you'll want the cogwheel to move upwards towards the roof of the car
good luck,
Craig