As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN CRUISE FLIGHT AT FL370 RECEIVED EICAS MESSAGE "CRG AFT FIRE DET FAIL". FWD CRG FIRE EXT SYS ALREADY ON MEL. QRH REQUIRES LANDING AT NEAREST SUITABLE AIRPORT, INFORMED ATC AND DECLARED AN EMERGENCY AFTER REQUESTING A LANDING IN STL. LANDING UNEVENTFUL. MAINTENANCE REMOVED AND REPLACED THE LOW RATE CARGO FIRE BOTTLE.