Memory card? Are you talking about the Wii's SD card?

You can't choose a SD card to format in Wii U mode, maybe you're talking about USB sticks.

Some USB sticks work, others don't. They're not recommended by Nintendo. Yeah, the formatting process never stops with incompatible devices.

Slow USB sticks can cause crashes, external hard drives usually run at the USB 2.0 limits.

The SD card reader at the front is only there for Wii backwards compatibility. Only usb storage is usable by the WiiU.

Not sure what you formated...

I use a 32GB USB stick for one of my Wii Us. It works fine.
For my other Wii U I took a Seagate 500 GB HDD from a broken laptop and placed it in a case I bought for $20. I have only used it with a Y-cable (came with the case). The Y-cable doubles the current to the HDD. It is plugged into both USB contacts in the back.

TIP: If you have old USB sticks with only 2 or 4 GB that you do not use anymore, you can use them for smaller games or save data. Then you can have them lying around, and change them when you change game. 

Does the WiiU not come with a users manual? Maybe that is how they kept the price down...

On the main menu, the one that appears when you press the home button, is a "button" for manuals. There is the instructions for the use of memories and other stuff. 

What is cool with this button is that if you play a game and go to the manual-button there will be the digital manual for the game. I guess a lot of people miss that.

Go buy a 2TB for $65 on eBay.

For Wii U: Plug it into the back port(s.) If it has an A/C adapter, the top one will do fine. If you need to use a Y-Cable because you have a pocket-sized hdd, use both back ports with the data cable on top.

For vWii: Running homebrew applications such as WIIMC (though I HIGHLY recommend setting up a PLEX Server instead and running through the Wii U Internet Browser in 1080p) or even homebrew games like WiiCraft, use a 32GB SD card. If you have a lot of movies and TV Shows, use your hard drive using the aforementioned setup depending on the hdd you are utilizing.

It is really that simple.

Only way to use SD cards for storing Wii U memory is with one of those USB SD card reader

see. THIS is why u don't get free games from me. A portable HDD of virtually any size (smaller than 3 TB) should work. It'll probably need a USB splitter cable, though. Got mine from amazon.com.