A few months ago, Nintendo posted a list of hard drives that have been confirmed to work with the Wii U. At the time, there were only 4 on the list. Now, the Big N has added a few more products that have been "subjected to basic peformance tests," bringing the total to 10. Here is the full list, straight from the official Nintendo website:
We have found that the following external storage devices work with the Wii U system; these device(s) have been subjected to basic performance tests:
1 TB: HD-LB1.0TU3-US
2 TB: HD-LB2.0TU3-US
Backup Plus Desktop Drive
1TB : STCA1000100 (US)
2TB : STCA2000100 (US)
1TB : STBV1000100 (US)
2TB : STBV2000100 (US)
1 TB: HDWC110XK3J1
2 TB: HDWC120XK3J1
Western Digital (WD)
1TB : WDBACW0010HBK
2TB : WDBACW0020HBK
I would like to emphasize that these are not the only hard drives that will work with the Wii U. They are simply a few that Nintendo themselves have tested, and they state that they "will expand this list as more devices are confirmed to work with Wii U." In addition, they have posted quite a few disclaimers:
We will expand this list as more devices are confirmed to work with Wii U.
To optimize performance and ensure a consistent experience, all USB storage devices must be formatted and configured for use with the Wii U console. Once a device has been configured for use with a Wii U console, it cannot be used with another Wii U console, PC, or MAC unless it is reformatted. Please be sure to back up any content before formatting the hard drive for Wii U as this content cannot be recovered.
Please note that we do not guarantee the performance of the devices.
Specifications of these devices are subject to change, and there is a possibility that these devices may not function properly with the Wii U system due to a specification change.
Please contact the manufacturer of the USB hard disc drive if there are technical issues or problems with your USB hard disc drive.
Amazon.com has listings for these hard drives on their website. You can check them out via the following links:
BUFFALO DriveStation Axis 1 TB USB 3.0 Desktop Hard Drive - HD-LB1.0TU3
BUFFALO DriveStation Axis 2 TB USB 3.0 Desktop Hard Drive - HD-LB2.0TU3
Seagate Backup Plus 1 TB USB 3.0 Desktop External Hard Drive STCA1000100
Seagate Backup Plus 2 TB USB 3.0 Desktop External Hard Drive STCA2000100
Seagate Expansion 1 TB USB 3.0 Desktop External Hard Drive STBV1000100
Seagate Expansion 2 TB USB 3.0 Desktop External Hard Drive STBV2000100
Toshiba 1TB Canvio Desk Desktop External Hard Drive (Black/Black) (HDWC110XK3J1)
Toshiba 2TB Canvio Desk Desktop External Hard Drive (Black/Black) (HDWC120XK3J1)
WD My Book 1TB External Hard Drive Storage USB 3.0 File Backup and Storage
WD My Book 2TB External Hard Drive Storage USB 3.0 File Backup and Storage
Nintendo notes that many of these products can also be found at these other online retailers:
Should you decide to purchase a storage device not on the list, here are a few things you need to keep in mind.
For a USB drive to work with the Wii U, it needs to:
1. be powered by an external power adaptor (a power outlet)
2. use a Y-cable to draw power from two USB ports on the Wii U
Warning about USB flash drives
Nintendo warns users that while the Wii U can detect USB thumb drives and flash storage, they are not recommended due to the fact that flash storage has a maximum number of reads/writes, which leads to data loss over time.
The Wii U will only read up to 2TB of storage even if an external hard drive has more space.
Source: Nintendo Direct