fried my arduino mega, well two of them in 2 minutes :S (1 reply)

Hi

I'm building a delta 3D printer.
Electronics are 2 kits from gadgets3d.com bought on ebay.(arduino mega, ramps 1.4, sdramps, G3D drivers.....)
Power supply is 12V 30A, switching...

I tested tested the motors couple of weeks ago, everything fine.
[www.youtube.com]

So I guess, this far everything was connected correctly.

On saturday evening, I put everything in the printer, connected ramps to power outlet and tried to connect USB cable to arduino, and as I just touched USB port on arduino SMOKE from arduino X(
Only the motors and endstops(two wires only) where connected.
SMD element just above the USB port blew. (diode !?, or what is it ?)

Tried the second board, with only the motors connected and same thing happend.

Next day I tried the first board again, without anything connected and now it's not detected by windows anymore, before I could still upload firmware to first board.

Second board has still demo firmware on it, is detected by windows, but I can't update firmware.
It works in demo mode, but that's it.
[www.youtube.com]

As I have AVR ISP for programming my quadcopters, I will try to program arduino this way, just for fun of it.....

I have two Arduino MEGA's on order, but would hate to do the same mistake again.

Can anyone more experienced tell me where did I go wrong, please ?