title_body: One motor on quadcopter broken I have a small quadcopter and I realised one of the motors does not turn electronically, meaning it can't fly properly. There is no visible sign of something stuck in the motor, like some string or something. What can I do to diagnose/fix the issue?
upvoted_answer: So this method will vary based on whether they are brushed or brushless motors.
  
  For brushed:
  
  Spin the propeller with your fingers - is anything actively resisting the motion?
  
  If yes, remove the prop and check for something not immediately visible, like a hair or carpet fibres caught around the motor shaft.
  
  Remove anything you find and try the prop again.
  
  If nothing is resisting motion, check the connections between the FC and the motor.
  
  If all else fails, swap the motor with the one diagonal from it. If the problem travels, the motor is at fault. If not, there is trouble with the board.
  
  It is worth noting that brushed motors are a consumable - the brushes wear out, and most aren’t rated for more than a number of hours’ use - see the diagram below:
  
  
  
  (Image Source)
  
  If the motor is brushless, start by again turning the motor, and check no screws are touching the windings.
  
  Check all motor connections and see if there are any breaks.
  
  If there is not active resistance, swap over two of the motors. If the problem follows the motor, it is likely a problem with the motor. If not, there is a problem with one of the ESCs.
  
  If you do encounter active resistance, look inside the bell and see if any of the magnets have slipped. If they have, use epoxy and tweezers (though a toothpick also works well) to glue the magnet back in place. I’d recommend also buying a new motor as it is now likely to be slightly unbalanced which may cause oscillations.
  
  See diagram of brushless motor: 
  
  (Image Source)