DIAGNOSIS: A loud mechanical grinding from the rear motor housing indicates a physical obstruction or internal damage, most commonly a failed wheel bearing or debris caught between the brake disc and caliper.
FIX: 1. Lift the rear wheel off the ground and spin it by hand. Listen and feel for the grinding. 2. Visually inspect the gap between the brake rotor and the silver caliper. Remove any visible stones, sticks, or metal fragments. 3. Check that the brake caliper is aligned and not rubbing constantly on the rotor.
IF THAT DIDN'T WORK: The motor's internal planetary gear reduction system may have worn or damaged nylon gears. This requires disassembly of the motor or replacement.
HEADS UP: Continued riding with a grinding noise from the motor can permanently damage the gearbox or seize the wheel. Stop riding until the source is identified and resolved.