If you have recently updated firmware on your Rev F Printrboard or installed the G2, you may find that your X, Y, Z and/or extruder are moving in the opposite direction that they should. Throughout the span of the life of our product line, there have been a variety of motor wiring orientations and firmware versions. If you end up with the wrong combination of wiring and firmware, you will see strange behavior from your Printrbot, i.e; X axis homing in the wrong direction - resulting in a grinding noise, or drive gear retracting instead of extruding.
This is a problem easily and quickly solved.
Try reversing the wiring 180 degrees so that the colors are opposite. For example, if your X axis motor wiring is plugged into "X-MOT" on the Printrboard and the colors are red/green/yellow/blue left to right. Unplug the motor wiring and plug it back into X-MOT blue/yellow/green/red left to right.
Another way to solve this is reversing the pin order on your motor wiring. See the video below for more.