If the Wii U says "USB device could not be formatted" even with different filesystems and a Y-cable, the problem is usually one of these:
* not enough power to the drive
* incompatible USB enclosure/adapter
* drive connected to the wrong USB ports
* the drive has partition issues the Wii U does not like
A few things to try:
* Use the rear USB ports only.
With a Y-cable:
* the data plug should go into the top rear port
* the power-only plug into the bottom rear port
* Completely wipe the drive partitions on PC first:
* Windows Disk Management → delete ALL partitions
* leave the drive as completely unallocated space
* do NOT manually format it afterward
* Some USB flash drives and cheap SATA adapters simply do not work properly with Wii U. External HDDs with their own power supply are usually the most reliable.
* Avoid NTFS/exFAT for Wii U storage. The Wii U formats the drive itself into its own filesystem anyway.
* Try another enclosure/USB adapter if possible. A lot of formatting failures are actually USB bridge incompatibility issues.
* If the drive is larger than 2 TB, compatibility can also become problematic.
One more thing:
If you are using a front USB port, move it to the back. The front ports are notoriously unreliable for HDD use on Wii U.
