I bought a Mega 2560 on Ebay (silly me). It was very cheap. Three weeks later (after the 14-day return warranty had run out) I connected it to my little Acer laptop using the USB cable. The Acer suddenly died and wouldn't restart after I had unplugged the Mega. I got worried. I found that I could get the Acer booted after I had removed and re-installed its battery. I thought the Mega might be taking too much power from the little Acer and it had tripped out the Acer. So I plugged the Mega into my bigger Dell laptop (silly me). The Dell died immediately and has never booted beyond the blue screen of death. Guess I will be dumping a cheap Mega and a Dell.
Question: I have noticed that when I plug the wall power supply into the Mega it flashes two fast and then continues to flash slowly. My Uno flashes four fast flashes then one slow flash when I plug in the power. Are these diagnostic codes? What do they mean?