I am having this issue aswell.So it failed to install Xenoblade Chronicles X, Resident Evil: Revelations and Lego City Undercover, now whenever i get into the storage to see what games i have installed... the system tells me that there is unnecessary data and if i want to delete it... i hit delete and then its been 9 hours and still says the same, what is going on? did i screw up my Wii U?
That happened on the WII U internal memory. When I hit external storage still say the same.