Just pullin this out my ass here, but have you tried switching the plugs on the Y cable?
Also, I thought I read that Nintendont doesn't like both it's holes filled. Maybe run one plug of the Y cable to a different device for power?
I tried powering it from a iPhone charger and just leaving the data in in the Wii U and YES! IT WORKS.
Looks as if the Wii U just isn't powerful enough USB load to run this HDD with a Y-Cable.













