I've spent the past week and a half softmodding my Wii U and ripping my library to it.
There are a few reasons for this -- the primary one being that the copy of Breath of the Wild that I bought used worked for about the first ten hours of the game and then quit reading.
Another reason is, it'll be nice to be able to put all my discs in a box somewhere and get some shelf space back.
The guide at wiiu.guide is a great walkthrough for softmodding your Wii U. But there are a few details I had to figure out myself, and I'm going to share them here.
First of all, here's the hardware I used:
A 1TB Western Digital Elements USB3 hard drive. This is excessive; I have 11 games installed on it and they only take up about 90 GB of space. However, I happened to have it lying around unused (I'd bought it for my grandma as a backup drive and discovered, when I went over to her house, that she already had a backup drive), so that's what I went with.
Here are a few things I discovered along the way:
It's probably a good idea to repartition and reformat the SD card before you get started. I found that mine had a few MB of unpartitioned space at the beginning, and I got an error with the NAND backup program saying it wasn't a FAT32 disk.
Also, make sure the FS type is C. That's FAT32. I used mkdosfs and wound up with 7 (ExFAT).
Something to note about the hard drive: I didn't need the Y-connector that wiiu.guide recommends, but I did need to plug it into one of the USB ports on the back of the console (I went with the top one). When I plugged it into one of the front USB ports, it would frequently hang on long file copies. When I plugged it into the top back port, it worked fine.
Copy all your save data before you rip any games. By this I mean, as soon as you format the hard drive to Wii U format, go into Wii Settings and Data Management, and copy all your save data. (It's safer to copy it than to move it; if you want to delete it from your NAND, wait until you've made sure it works first. A NAND backup and SaveMii backups are probably a good idea too, just to make sure you don't lose anything.)
This is totally counterintuitive, but here's how it works: save data on the NAND works for disc games (and, presumably, games stored on the NAND, though I haven't verified this), but games installed on the hard drive will completely ignore it. If you've got Breath of the Wild installed on your hard drive, and a saved game and a few gigs of updates installed on the NAND, then when you fire up Breath of the Wild it will behave as if it's being run for the first time. It will try to download updates, and start you out at the beginning. If you want a game that's installed on your hard drive to see your updates and your saves, then they have to be stored on the hard drive too, not the NAND.
And, even more counterintuitively, you have to copy the saves first. If you install the game on your hard drive and then copy the save data over, the save data will overwrite the game on the hard drive and you'll have to reinstall it. But if you copy the save data and then install the game, the game won't overwrite the save; the save will still be there and the first time you run the game off the hard drive, all your save data, updates, and DLC will be there, ready to go.
Hope that helps somebody. It would have saved me a lot of extra hours if I'd known that stuff before I started instead of having to figure it out for myself.