Ok, since there are 2 rear usb ports, you swapped the usb Y cables around as well?
If it still doesnt work, like others have stated, looks like whatever usb controller that is inside the hdd enclosure is not compatible with the WiiU.
In the past I had various hdd enclosures and spare hdd lying around, my WiiU couldnt read some of them, even though on my PC they read fine.
Added Notes:
Just a noob question, Im hoping your not connecting the hdd while the WiiU is on?
WiiU should be off, connect hdd and then turn WiiU on.