Hello! First of all, sorry for my bad english.
I have 4WD Arduino Mobile Platform(http://www.rugcommunity.org/page/dfrobot-4wd-arduino-mobile
), Arduino Mega 1280, Motorshield v.2(2 servos and 4 dc motors), 5x 1.5AA Alkaline batteries for motorshield and 9V for my Arduino board. Power is separate. All works fine but when i try to run my dc motors(via usb), the motors can stop me disobey. Its a big problem for me, coz I can't control my mobile platform...
Why it happened with my hardware?? Help me pls.