One of the greatest features of the Wii U is its robust offerings within the Nintendo eShop. Whether it's classic gaming (Punch-Out!!) or modern titles (Assassin's Creed 3!), the eShop is an easy way to cut down on cartridge storage and acquire new titles quickly without bothering with a pre-order or traveling to the nearest big-box retailer.
But, one of the weakest features of the Wii U is its minuscule local storage. The basic model - which costs $299 and touts 8GB of memory - really only leaves about 3GB left over for game storage. The $349 premium model fares a little better, offering 25GB of storage, but could still hit capacity if you download a huge title like the 16GB monster Tekken Tag Tournament 2.
The good news is that you can compliment your console with a USB-connected external hardware drive, but it does have to fit certain requirements and formatting to work well with your Wii U.
Please keep in mind that only a handful of these products are actually recommended by Nintendo for use with the Wii U, so plug in at your own risk.

Nintendo-Endorsed Desktop Drives:

In the market for a hard drive? Nintendo has a short list of recommended products that are proven to play nicely with the Wii U software.
Because of the behavior of the console, Nintendo has a few recommended parameters for an attached hard drive. First and foremost, the manufacturer advises to use an A/C-powered desktop hard drive rather than a USB-powered portable drive or a solid state drive to ensure that adequate power is distributed and memory is accessed quickly. Next, the company limits the size of the external storage to 2 TB, so the console won't read any mega-memory monoliths (though Nintendo has promised that will change sometime in the future.)
After narrowing down to a handful od hard-drive options, Nintendo has personally backed five of them for use: Buffalo Drive Station Axis , Seagate Backup Plus, Seagate Expansion Desktop , Toshiba Canvio Desk, and Western Digital My Book. Nintendo doesn't guarantee that any of these specific drives will perform optimally on the Wii U, but they are all tested for success.
If you're in the market for a hard drive specifically for the Wii U, and you don't care much for customization, then picking one of these up is best. They'll set you back $100, and you'll have the peace of mind that comes with a company-endorsed product.

Other Desktop Drives:

It's fair to say that not all hard drives are the same, so it may be worth it to try a few other options if none of these pique interest.
The slightly older (and cheaper) Western Digital Elements 2 TB Hard Drive utilizes USB 2.0, which matches the Wii U's output. This hard drive can be picked up for cheaper than its brethren, largely due to the fact that it doesn't have any of WD's signature backup automation. But, none of that matters when it's formatted for the Wii U, so it's an opportunity to get the biggest bang for your buck.
The stylish and power-conserving Iomega Prestige 1 TB or 2TB Hard Drive is another USB 2.0/3.0 hybrid is also an option to consider. When not in use, the hard drive shuts down to conserve power, so no vampire power will happen under the guide of keeping your downloadable library safe. But, ensure that the hard drive does turn on with your Wii U -- the console doesn't have "hot swap" capabilities, so it won't recognize the hard drive once it's fully powered up.
Safety freaks and downloadable game hoarders may want to spend a little extra to pick up the IoSafe Solo 2 TB Fireproof and Waterproof External Hard Drive. Sure, it's about $100 more expensive than a standard hard drive, but it's a trade-off for getting an iron fortress of storage. The IoSafe Solo is fireproof up to 1500 degrees Fahrenheit and waterproof up to 10ft for 72 hours, so it can withstand any natural or man-made disaster.
But use at your own risk — none of these hard drives are guaranteed to work smoothly with the Wii U. That doesn't deny the fact that they are great hard drive options, so go for it if the features fit your needs.

USB-Powered Portable Drives:

Nintendo doesn't recommend USB-powered portable hard drives for the Wii U, but that doesn't mean it's not possible. Because the Wii U has USB 2.0 drives, portable drives need a Y adaptor plugged into two ports to ensure optimum performance.
There are many great portable drives out there, but a short list of the best include the Western Digital My Passport, LaCie Rikiki USB 3.0, and the Buffalo MiniStation Plus. The benefit of similar performance to a desktop drive in a much smaller size really shows off the appeal of a portable drive for the Wii U; it's just so nicely inconspicuous. It's also much more likely that you have a spare portable drive laying around, so don't be afraid to test it out with your console.
In addition to the power requirements, it's key to recognize that once any hard drive is configured for the Wii U, it cannot continue to hold computer files. However, a stack of portable drives that hold games may be a smart way to get around that pesky 2TB limit.

SD Cards and USB Sticks:
This is the absolute last resort, but if you find that you've run out of storage options and you must download a game immediately, it is possible to hook up both a USB flash drive and an SD card to free up memory.
Flash drives can be recognized by the Wii U and don't need any special adaptors to work correctly, but Nintendo strongly advises against them due to the constant read/write processes that happen during games. You can use a USB stick for game storage, though, and you can simply transfer games from the stick to a much sturdier drive when the time is right.
An SD card may be used to free up space on the Wii U, but not for games. Nintendo says that they're only able to store Mii-related information and photos, so don't try to use one for game storage.
The Wii U has storage issues, but there's plenty of ways to get around it. Be open to products that suit your needs, and you'll be able to download as many games as your heart desires.

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