I too got a “bad” Arduino Mega with my first parts bundle. I had another Arduino Mega (original) that worked fine so just put the “bad” one off to the side and used mine. While building the next MPCNC, I again picked up the “bad” one… and again it didn’t work. Not having another on hand at the time, I noted that it was a “clone” and googled around a bit and found that many cheap clones do not use the standard serial to usb chip original Arduinos use. They use a chip marked CH340 and your computer will not recognize these CH340 chips until you download a proper driver. The following Instructable (and several other sites) describes the problem and fix… it worked for me and I’ve had no problem since.