A friend of mine asked for a couple files I had on my MAC. So me being the friend I am I said sure. He brought me his USB drive and I copied over the files he needed. Unmounted the drive and thought no more of it. Today he called me saying the drive is asking to be formated on Windows, but all I did was copy a few files over, is there something I can do to fix this issue without him lossing his data on that drive?