Ok. I tried to change my REAR pads and disks today. I got the wheel off and the caliper, but I don't know how to take the rotor off. Looks like there is some big iron cover with 2 notches in the middle of the rotor but I couldn't figure out how to unscrew it. Can someone advise me. If I need a special tool is it available at local auto stores and if so what is it called and which store? My pads are shot so I have to get this done. Thanks.

Mike