Another reason to do pre-flight bench tests. One motor failed to spin up.

[Sureshot]

Prior flight had no problems. Four days later, doing a quick bench test, the left rear motor shakes & buzzes, no spin.
Of course the ST16 messaged 5 motor mode.
The motor wound spin manually, but would not when tested with the GUI.
I feared it might be a bad ESC, but decided to swap the motor first.
I removed the arm to swap it out then noticed the motor would jam when turned manually.
I found a tiny metal chip stuck on the motor magnets and remover it with non-magnetic tweezers.
While the arm was still hanging in the wires, I connected the GUI and it worked.
After re-assembling & testing, I flew the mission.

 

[Mrgs1]

Fragment from motor or external?

 

[Sureshot]

Fragment from motor or external?

Something from outside, maybe from a steel cutting saw?
I was confused at first since I could hand spin the motor.
The debris would only jam the rotor when the stator was energized so it looked like a ESC fail.
The shaky movements involved in removing the arm exposed the real culprit.