Would it harm the arduino when I connect both USB and the 2.1mm center-positive plug at the same time? I can imagine that some shorts will occur when connecting it to two power sources. For my project it will be powered via the 2.1mm center-positive plug, but for easy access to the code I want to have a USB connector build into the chassis and a cable from there to the arduino. This way I do not have to open the chassis to update the code.