I've tried a USB thumb drive and an external hard drive and they show up without any files but once windows files are on it they aren't recognized. I ran bootcamp and did the whole partition and then it goes to a gray screen. It just sits on the gray screen so I power if off and restart i get a black screen saying unable to find boot device. I restart holding option key and i only get mac hard drive and recovery. I have a 2010 MacBook Pro running mavericks. Any ideas?