Hi,
At the moment I've just got a couple of Wii U games installed on the internal 32GB and a 64GB USB flash drive with a few Wii games on. I haven't been able to play the latter yet as I haven't got the right controllers yet but they all loaded OK when I tested them.
Even so, you may be right. I see there appears to be a way to use just one HDD and make the Wii games VC injects now
https://gbatemp.net/threads/run-wii-wiiu-and-gc-backups-off-the-same-external-hd.489718/
Even so I still need to know if I can just use 2TB of a 4TB HDD as a Wii U partition and use the other 2TB as an NTFS partition? I doubt I'll need more than 2TB for all the Wii U/Wii/Gamecube/Emulator games I want to install, so if I can't split a 4TB HDD it's pointless buying one and I might as well just get a 2TB drive.