Wii U is not strong enough to power a hdd drive, you need to connect the hdd to your wii u, and also to an external power source, alternatively you can connect your hdd to both usb slots, then it will power the hdd and it will work

if it's already powered via a y splitter or an external power source, try reformatting the drive through the system settings app to format it to the wii u's propritary format type if you're going to be downloading games FROM the console, as the wii cant read fat32 for ext. drives