I posted about this tip a few months ago but it's a pretty useful hidden feature so I'll post it here so everyone can know how to do it. In the System Settings go to Data Management. Once there don't touch any of the on-screen buttons but instead hold down on the D-pad and the "+" and "-" buttons for a couple of seconds. A hidden option will show up to enable or disable the Sleep Mode on the removable HDD. You can do this even if you don't have a HDD plugged in the Wii U.