As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB AT APPROXIMATELY 6500FT, CREW RECEIVED AFT CARGO DOOR MASTER WARNING. CREW NOTICED THAT THEY WERE LOSING PRESSURIZATION. CREW LEVELED OFF AT 7000FT AND REQUESTED VECTORS BACK TO YYZ. THE FLIGHT LANDED WITHOUT FURTHER INCIDENT AND THE AIRCRAFT WAS MET BY A MECHANIC AT THE GATE. MECHANIC NOTICED THAT THE CARGO DOOR HANDLE WAS POPPED OUT.