Normaly a hdd is best choise for holding data. A not in use hdd will hold them for more then over 10 years, if nothing special happens.
Keep games as Install files:
The price for a Backup HDD is very low. If u backup the wud files for reinstall on an 3,5" HDD, the actaul cost will be around 65€ for a 2TB ( seagate baracuda for example, or about 90€ for 4TB). A externel 2,5" USB3.0 ist more expensive.
Clone the Drive:
U can copy the Games from one Drive to another with the Wii U function.
Because of the own Format of the Wiiu u need a HDD with same Partition size of your Gamedrive, u cant select a specific Partiton on the Wiiu.
There are two way of getting still the wii u games.
1. Using Wiiu USB Helper with aworkaround for getting Backups of your Games for new install
gbatemp.net/threads/working-fix-easy-install-wii-u-usb-helper-0-6-1-655.535214/
2. Or using a different site for your Games. Google and [Edited] will help u.