What was found
EWR/BTV---CO3526, ON TAXI, CREW REPORTED APU FAIL AND SMOKE ENTERED COCKPIT.....AIRCRAFT RETURNED TO GATE UNEVENTFULLY. MAINTENANCE INSPECTED AIRCRAFT/APU AND FOUND APU SHAFT TURBINE NOT TO SPIN. RAN BOTH ENGINES #1 AND #2. NO SMOKE IN COCKPIT. ALL SYSTEMS CHECKED GOOD. AIRCRAFT RELEASED IAW MEL 49-00-00 CONTROL NUMBER 023128 APU INOPERATIVE. AIRCRAFT RETURNED TO SERVICE.....BTV CO3387, MAINTENANCE REMOVED AND REPLACED APU IAW EMB145 AMM #49-10-01. OPS. AND LEAK CHECKED GOOD. MEL CLEARED AND AIRCRAFT RETURNED TO SERVICE.