According to Wii U Internal Storage Space Information (Nintendo Wii U FAQ):
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 this in mind, my theory is to partition a 4TB or 5TB hard drive into 2TB each partition, so if I connect a single disk, wii U ... (in theory) Wii U should recognize those 2 partitions of 2TB each. But I don't know if this works and I would like to know if anyone has tried before. Some have already answered similar questions such as that it might sound insane to have more than 2TB when the Wii U romset weighs only 750GB (including DLCs + Updates). but I also wanted to add the Wii (2TB) and Gamecube (600 GB) romsets through Wup Installer in the main menu. I calculated the total and with 3.7 TB it would be enough.
it really is a 5TB disk (specifically ADATA HD710, that I have not yet bought) but due to the format, surely some GBs are lost in the manufacturing process and they are really only 4.5TB, and I would like them to be at least partitions of 1990GB.
Additionally, I don't have any more discs to do the tests but I would like someone to try it with an HDD of approximately 120GB, partitioning into 2 (60GB each) and telling me their results.