As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFT CARGO FIRE EICAS WARNING ANNUNICATED AFTER LEVELING OFF AT FL 340. DECLARED EMERGENCY, JETTISONED FUEL, RETURNED TO DEPARTURE. AFT CARGO FIRE LOOP A FAILED TEST. FOUND FAILED DUE TO MOISTURE AND FOG. SWAPPED SMOKE DETECTOR TO FWD CARGO LOOP AND BOTH DETECTORS MAKE WARM AIR AND REMOVED MOISTURE. RESET SMOKE CARGO CIRCUIT BREAKER. GROUND TESET PASSED. MESSAGE CLEARED. IAW AMM 26-16--1 AMD FIM 26-16.