You can connect the WiiU drive on port0 (Top port) and
nintendont will work with the two HDD connected.
But users reported issue with USBloaders and cIOS v10-alt not working as intended with port1.
It's not a feature that
nintendont lets you use Port1, it's only a result of different updates done recently.
If you don't use USBLoaders, it's a good workaround : exchange port1 and port0.
If you use USBLoaders, it's not a solution.
I re-added the warning on the first post telling that DUAL HDD is not supported.
It shouldn't corrupt savegame anymore, but I left it just to be careful.