Enable HPA to usb??

Host Protected Area?

 

You can't have multiple partitions on a Wii U HDD, actually if you try to modify the partition table at all it will just ask you to reformat.
Edit: Oh, I see what you mean now. You want to hide one part of the drive when in the Wii U OS, and hide the other part when you're in vWii. I have no idea if that would work or not, but anyway, it sounds like quite a lot of hassle, if you have to keep plugging it into the PC to switch which partition you want visible. It probably wouldn't work though, as you still need a partition table with both partitions listed in it for both Wii U and vWIi to be able to see it. And as said the Wii U OS would just ask you to reformat if you try to modify the partition table. HPA works because the data doesn't necessarily need to be a partition, and it can be hardcoded to treat a certain section as say a fat32 or ext4 partition if necessary, but you'd have to code that functionality into the Wii U OS and into cIOS yourself for it to be able to mount the hidden area, without needing a modified partition table.