Good Morning.
View attachment 249695
A "simple" Explanation:
The Wii U has a "not so good" USB Power Supply/Voltage.
That is the Reason why you should not really use USB Sticks/Pen Drives/Thumb Drives/Flash Drives and HDD with a single USB Connection.
It is also depending which Controller(Chip) is used in the USB Device(s).
For the Wii for Example "older" USB Sticks are sometimes better working than Newer.
For the Wii U - a HDD with Power Supply and an Y-Cable is a very good Choice.
For the vWii - honestly the same.
But or course,some Devices are working without that Recommendation Setup.
Thank you.