Where to connect an external power source for an Arduino motor driver shield to?
There are two things you need to do:


Prevent the motor drawing its power from the micro-controller board.
Make sure that all the grounds are connected but avoiding motor current return through the micro-controller board.








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A good wiring configuration. The heavy wires carry the motor current. 

Note that there is one ground connection between the micro-controller and the motor system. The return current for the 12 V supply does not pass through the micro-controller board.