What was found
(CAN) THE A/C WAS DISPATCHED WITH PACK NR 1 INOPERATIVE IAW MEL. AFTER T/O, WHEN TRANSFERRING THE BLEED AIR FROM APU TO THE ENGINES, CABIN ALT CAUTION INDICATION CAME ON ON THE EICAS. THE CREW BEGAN TO FOLLOW QRH PROCEDURE AND, SUDDENLY, CABIN ALT WARNING INDICATION CAME ON ON THE EICAS. THE CREW BEGAN AN EMERGENCY DESCENT AND THEY RETURNED TO THE DEPARTURE AIRPORT. A/C LANDED UNEVENTFULLY. DUCT FROM PRECOOLER TO PRIMARY HEAT EXCHANGER IN PACK NR 2 WAS FOUND MISS ATTACHED AT PRIMARY HEAT EXCHANGER SIDE. DUCT REPOSITIONED. AMM TASK NR 2 AMM NR 21-31-00-710-801 CARRIED OUT. A/C RELEASED INTO SERVICE.