Since you should disconnect power when changing pololu trimpot, you can disconnect the unused motors as wel.
You should connect / disconnect motors only when it is not powered too.
Edit: Aslo even when steppers are not moving they consume power. So if you wanted to measure it on the power source there is no way except disconnecting all the other motors. If you wanted to use this approach, then loading cartesian firmware would not help. Other good approach is to measure voltage on the pololu trimpot and compute current from that voltage.
You should connect / disconnect motors only when it is not powered too.
Edit: Aslo even when steppers are not moving they consume power. So if you wanted to measure it on the power source there is no way except disconnecting all the other motors. If you wanted to use this approach, then loading cartesian firmware would not help. Other good approach is to measure voltage on the pololu trimpot and compute current from that voltage.
