Causes
An interruption in the network can cause the whole system to freeze up. To test if this may be your issue, disconnect the system from Wi-Fi and try playing a game that is connected to MiiVerse to see if freezing continues. If it does, change your router settings from “N” to “G”. If you do not know how to change router settings, try using a Wii LAN adapter to connect an Ethernet cable to your modem or router.
Certain external hard drives, such as ones that use the console as as the main power source, can cause freezing in games. Even if your external hard drive uses an external power supply, it can still take power from the USB and overwhelm the Wii U.
Try to move the titles that are freezing from your external hard drive to the Wii U’s internal flash storage. Then, unplug the external hard drive and see if the moved titles work now that they are in the Wii U’s internal flash storage. If they work fine, then the particular external hard drive you own is causing your system to freeze.
Certain devices that you plug into the USB port (keyboards, chargers, flash drives) can drain the Wii U’s power source. Unplug all devices plugged into Wii U’s USB ports to see if the freezing issue stops. If it does, you can individually test each device to see if a certain one is causing the system to freeze.