Pololu Ground problem on Rev 4 boards
There is a problem with (all?) Rev4 boards: GND on three of the 4 Pololu's is not connected to the main GND of the board. This is a severe problem and it can blow up your MBED or Pololu!
Still need to find out where in the production process this went wrong and if it applies to all Rev4 boards.
To check if your board has this problem, chek if there is connection between GND of the Pololu's X,Y and E and the GND of the power connector. If not, connect it, for example by making a bridge on the backside between the Pololu GND of the E-Axis and the GND pin of capacitor C3 (near the power connection).