Hm, that's odd. Mine automatically detects the external harddrive and downloads stuff to it instead of the system memory. I have a basic Wii U as well.
Though I did have another external harddrive before my current one, which was giving me problems because it didn't receive enough power. Make sure it either has its own power source, or use a Y cable to connect it to two USB ports on your Wii U (not just 1), if you haven't already. That way it gets enough juice.
Other than that, I have no idea what the problem could be; there are no settings you could change on your Wii U to make it download to your external harddrive, you can only move/copy/delete data between it and your system memory...
If all fails, contact Nintendo!
Nintendo Network ID: Cheebee 3DS Code: 2320 - 6113 - 9046
