Can't Remove Diamond Grinder Cup
Put the grinder on a concrete surface like you're going to grind it, hold the lock button to prevent the shaft from spinning, push down into the concrete and turn the grinder counter clockwise to unscrew the diamond cup. 

If that doesn't work, do the same thing with the diamond cup on concrete but stand with your left foot above the diamond cup on the backside of the grinder, and kick the other end (power cord side) forward with your right foot. You need to press the shaft lock at the same time, but sometimes you can get it to stay locked or use a broom handle or something long to press it to keep the shaft locked when you kick.