A CRJ900 PLT RPTS DECLARING AN EMER AND RETURNING TO LAND BECAUSE A FORWARD CARGO DOOR WAS NOT CLOSED BEFORE TKOF.

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ON TKOF I NOTICED A LOUD LOW RUMBLE AND NOTICED THAT THE ACFT WAS NOT PRESSURIZING. I LEVELED OFF AT 2000 FT AND BEGAN A TURN TO THE N. I HAD MY FO CHK THE ECS PAGE AND COULDN'T DETERMINE WHY WE WERE NOT PRESSURIZING AND WHERE THE NOISE WAS COMING FROM. I TURNED FOR A DOWNWIND AND TOLD ATC THAT WE NEEDED TO RETURN TO THE ARPT AND DECLARED THE EMER. I HAD THE FO NOTIFY THE FLT ATTENDANTS THAT WE WERE RETURNING TO THE ARPT AND WOULD NOT EVAC THE ACFT. I MADE A NORMAL BASE TO FINAL LNDG. I RETURNED TO THE GATE AND SHUT THE ACFT DOWN. WHEN I WENT OUTSIDE I NOTICED THAT THE AFT; FORWARD CARGO DOOR WAS OPEN. I HAD THE PAX GET OFF AND DID NOT HAVE TIME TO TALK WITH THEM ABOUT THE EMER. I TOLD EVERYONE NOT TO TOUCH THE DOOR AND THEN CONTACTED OPS. RECOMMENDATIONS TO PREVENT THIS FROM HAPPENING IN THE FUTURE: 1)CHANGE MEL AND WT AND BAL MANIFEST TO REFLECT SAME DOOR TERMINOLOGY. THIS WAS A MAJOR FACTOR IN CAUSING CONFUSION IN THIS EVENT. 2) CHANGE MEL PROC TO REFLECT SAME PROC IN EMB MEL. ADDING THE LANGUAGE THAT REQUIRES FLT CREW AND GND CREW TO 'CAREFULLY CHK THAT ALL DOORS ARE CLOSED AND LOCKED BEFORE EACH TKOF' WOULD MINIMIZE THE RISK OF FUTURE OCCURRENCES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE CREW WAS CONFUSED BY THE MEL BECAUSE THE FORWARD AFT CARGO DOOR (CTR DOOR OF 3 CARGO DOORS) HAD AN AMBER EICAS ANNUNCIATION PRIOR TO THEIR PREVIOUS DEP BUT WAS CLOSED. THE CREW ASSUMED THAT WAS THE MEL'ED DOOR BECAUSE THE MEL'ED DOOR (AFT) ACTUALLY HAD A GREEN (CLOSED) INDICATION. FURTHERMORE; THE WT AND BAL DOES NOT REFLECT THE DOOR CONFIGN ON THIS ACFT AND THAT LED TO FURTHER CONFUSION WHEN THE DOOR CLOSED RPT WAS GIVEN. THE RPTR DOES NOT UNDERSTAND HOW THE PERSON RPTING CLOSED AND LOCKED COULD VISUALLY MISS AN OPEN DOOR BECAUSE THE DOOR ACTUALLY DROPS OUT AND DOWN; CLRLY VISIBLE BENEATH THE ACFT.