Today I had to call a guy and tell him, "you need pads and rotors."
"There's a grinding noise in the rear brakes" the customer told me when he dropped the bike off. I guess there is. Somehow, the retraction spring that holds the pads apart had gotten sucked in between the pads and rotor. It had been ground into the mechanism, scoring the disc. Then the rotor had rusted.
Disc brakes bring yet another jigsaw puzzle to the world of bikes. Rim brake pads are all made on a few basic formats. Not so, the proprietary disc pads. We had several types of pad in stock. We just didn't have the right pad. Only replaceable derailleur hangers have as much pointless variety.