As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI AFTER LEFT ENGINE START MASTER WARNING & MAIN CARGO DOOR LIGHT ILLUMINATED. A/C RETURNED TO THE GATE (BTB) AND MTX GROUNDED AC. FOUND MAIN CARGO DOOR WAS NOT COMPLETELY CLOSED CLOSED DOOR REMAINDER OF THE WAY LIGHT EXTINGUISHED IAW B767 AMM 52-32-00. MTX RETURNED AC TO SERVICE .