In all its simplicity:
If i install Copter 3.2.1, and try to callibrate accel, i will always get calibration failed.
If i install Copter 3.0, i get calibration success, but on flight data, i get HUD saying bad gyro health
IC2 voltage is 3.3
I had this once with a HK mini APM and it turned out to be exactly what the messages said it was, Bad Gyro’s.
It became evident when I left the board attached to the GCS for an hour or two and when I returned the artificial horizon was spinning like a top.
At least I received a refund from HK for it.