What if I don't (didn't ) cut the trace from VIN to VUSB when powering from an external 5V source? I made a dual CAN board with its own 5V supply powered by the vehicles 12V. This supplies the CAN transceiver and also the Teensy 3.6 when USB isn't connected. I'm using the SD card to log the CAN data. However, there are times when I would like to log via serial. So what's going to happen when I have competing 5V supplies? It's handy having the trace still in place as I'm still developing the code, but when it's loose in the wild I'd hate to blow anything up. I may still be able to get a hook in there and cut the trace as the Teensy is raised up from the CAN board.