I was expecting 10GB for Wii U, but 15GB?
I still don't have an external storage D: It's hard to find a cheap one and a Y cable on my location :/
A large chunk of the size is most likely the EU text for other countries.
For NA we can probably assume it's gonna be 12-13GB, maybe?
Y-Cables are ridiculously cheap online on Amazon and eBay and usually, getting your own SATA hard drive and throwing it in an enclosure is cheaper than a ready to go external hard drive.
If you already have an external and don't want a Y cable, Apricorn sells USB splitters that work for any USB output device. It works for the Wii U and pretty much any hard drive, but it's like 15 bucks...