Mapping letter? USB ports aren't assigned letters... unless you plug a storage device into them... but then it's the storage device that assigned a letter, not the USB port. What are you plugging in that isn't being recognized? If it is a storage device, plug it into another computer to make sure it works. If it does, then check what drive letters are already in use... there are some letters that USB storage devices won't grab. You may also want to check if you need drivers... which you shouldn't, but it is possible.