[my right]

I'll have to say I have never seen both disk and drum brakes on the same wheel. Usually the emergency brake is controlled by a cable and the brakes are activated by hydraulics, but both are activating the same shoe or pad.

[RobRoy]

I noticed it when I did my own brake job. Inside the hub (about 8" diameter) was a "mini" drum brake operated by the emergency brake.

From here: http://www.justanswer.com/questions/1g9ge-why-would-my-rear-rim-be-hot-to-touch-after-driving-2-hrs-on

The picture doesn't seem to work. Go to the link. They have a picture of exactly what I was talking about.