Can I use external stepper motor power and USB connection I have a RAMPS 1.4 with Arduino Mega 2560 with 2 steppers connected to an external power supply using Polulo stepper drivers. The steppers are for X and Y axis movement. This external power supply is set to 12 volts and power is applied. I have configured the marlin firmware also.

Can I connect my setup to my PC using USB when the external power is also on?
If i connect the USB while the external power is off, do the motors take power from USB? Will this will burn my board?
The motors are powered from 12/24V external power supply, so the motors will not be powered. 
the ramps Schematics shows the connection diagram and the power is provided via the VMot pin/12V.