I actually had the same problem you are having and I use the same hard drive with my Wii U. All I did to correct the issue was move ALL my game and save data to the hard drive. It seems when the drive is not accessed for a while, it will go into a standby mode. If this happens, and you try to return to the Wii U menu, it could freeze the console. Just make sure all game data is moved to the hard drive to the Wii U will have to access it more often via saving data or running games.
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