Tablets have great WiFi connectivity. If you get a NAS with a media player then you can access both the shared storage on the NAS and control the media player remotely and wireless from the tablet in your lap. No need to connect a harddrive directly to the tablet. All but impossible anyway.
You can then stream video from the NAS to the tablet, or music from the NAS to the stereo. And control it all from the tablet. You can also copy files from the NAS to the tablet over wifi so you can bring them with you.