If you made sure the drive board is connected correctly there is probably nothing that can be done (other than having a professional doing repair on the drive board and/or Wii U mainboard) if one is able to locate anything has gone bad.
There are some old Reddit posts about workarounds with varying success and some GBAtemp threads (as well a source I can't find at the moment where somebody claimed a CPU reflow did the trick – sounds strange, dangerous and complicated; definitely the very last thing to try if everything else fails).
It is never a bright idea to have different parts of a device being married to each other like the Wii U drive board and mainboard (look at recent Apple phones for WORSE
examples). Most of the time this is justified by the manufacturer with security or even DRM reasons. Really BS
, not future proof and prone to failure.
In case of the Wii U I would even call it a deliberate attack on the property of the user: The user paid for the Wii U and the hardware belongs 100% to the paying customer. In this case the paying customer can't use it anymore because a non-essential
part drive(board) or something processing data from the drive board on the mainboard failed. Having the OS complain, maybe even on every boot, but continuing normal operation after a "My drive is done for!"
with the remaining hardware would neither have compromised security (DRM) nor caused any harm on the user.
I would say they have no right to disable the rest of the Wii U if it can't connect to it's drive board.