The blower motor on my furnace would not start, it hummed. I tried to spin it to get it going but the motor was locked up (electrically, with no power it spins free). I heard that if you have a bad capacitor you can get the fan going with a spin - this is not the case here, as I said the motor seems to be electrically braking. I replaced the capacitor (not sure if it is a run or start) and the motor started. It ran for a couple days then quit again. I replaced the capacitor again and for another three days it seemed OK.

When a motor fails, it often makes the capacitor fail as well. Can you do an amperage check on the motor when it is running?