Get home tonight, to find that my rear barke pads sound like metal on metal as I roll my bike into the garage. Check to see what's going on and it looks like only one of the pads are worn right down to the metal. I had them changed out the 12000km service, so they couldn't possibly have worn down that quick (at 20000kms now), and I also just had an 18K service about 2 weeks ago, and the mechanic said nothing about the rear pads. what could have caused only 1 wearing down all the way? any help would be appreciated and now that's happened, am I up for a new disc aswell?