According to Wii U Internal Storage Space Information (Nintendo Wii U FAQ):
You can store games/content on an external USB HDD. "If you wish to download more content we recommend an external USB storage device with a dedicated power source so that you can expand your storage capabilities for Wii U software."
The storage device will be formatted for sole use by the Wii U. "To optimize performance and ensure a consistent experience, all USB storage devices must be formatted and configured on the Wii U console for exclusive use with the console. Once a device has been configured for use with a Wii U console, it cannot be used with another Wii U console, PC, etc., unless it is reformatted."
Maximum storage capacity for one device is 2TB. "Following the December  system update the Wii U will be able to recognize a storage device that is over 2TB, but 2TB will still be the maximum amount of storage capacity. Any extra capacity in the storage device beyond the 2TB will not be accessible via the Wii U."
With all of this in mind, is it possible to use a single storage device for both Wii U and non-WU use? For example, by partitioning it and having the Wii U format only one partition. Or does the second point imply that any partitions will be ignored and the entire device taken over in the formatting process? (In which case what happens to the remaining space on a >2TB device?)