Mongsa02

HELPPPPPPPPPPPPPPPPPPPPPP!!!!! When i use my MP4 it always says,"USB Device Not Recognized".At the front and back of the cpu USB slots,,,I reformat my Mp4 so it has no songs pics and vid's....And it does not charge if i use USB port,,but if i use my DC charger,it charges PLEASE HELP ME!!!!!!!!!!!!!!!!!!