Before you do any more troubleshooting with multiple variables, go into data managment and try copying a couple titles over to the USB drive. That way you're not dealing with anything other than a stock WiiU and a hard drive.
If that works, try installing a small title from SD to system memory, then move that new title to USB. If you're still good, remove the title.tik file from sd:/install/gamename and try installing that title again, but this time to USB. If small titles work, but large ones fail, you might still have bad downloads, a bad SD, bad usb drive, insufficient power/bad y cable, or sleep issues.