After hooking up my 1TB portable USB 3.0 drive up to the Wii U using the recommended Y-cable, I tried transferring Sonic All Stars Racing Transformed from the internal memory of the Wii U to the external drive. The whole process took just under 8 minutes to move 5.8GB of data.
Last time I tried moving files over without the Y-cable and I ran into errors, so it's definitely mandatory that one use a Y-cable for stable operation. Thankfully the cables are pretty cheap, so it's not a big deal.
I'm now looking forward to being able to download big games without worrying about storage, although with the 32GB internal memory of the deluxe it hasn't been an issue for me yet.