Can saved games be transferred from one profile to another on the Wii U?

As per NKCampbell's comment: Is there a way to transfer save data from one profile to another on the Wii U? Example: Transfer saved games of an old kid's profile (who earned the Master Sword in Breath of The Wild) to a younger child's profile?

Yeppers, as long as the folder containing the data to inject has been renamed to the correct user id (x000000x) this works flawlessly. I'm assuming this works by forcing the game's legit code to sign the save data to avoid hash mis-match? As O.P. stated, a direct copy using "SaveMii" for example, will result system hang and make it difficult to tear out the improperly signed data(I ended up using "FTPiiU_Everywhere" to manually delete the broken saves). As always: be careful a happy modding :-)

Thankfully this is no longer the case! No thanks to Nintendo....

There are a few ways to do it, but the critical caveat is that Saaviine must be used to copy the save to the new profile.

Here are the steps I followed. Depending on your level of computer competency there may be a bit of a learning curve but there are many good, easy to follow tutorials for each step of the process available online (gbatemp.net comes to mind, but google will help you :)

1. Jailbreak your WiiU!
2. Load the WiiU profile that you want to copy from.
3. On computer - Run Saaviine (https://github.com/Maschell/saviine/releases) in backup mode on your computer, and take note of what your computer's IP address is.
4. On WiiU - Run homebrew launcher and then run Saaviine. (or directly launch Saaviine if you want to / can). Saavine will require your IP address from step 3, make sure it is correct! If you are using a disc game, press A once the IP is configured, if the game is not installed on disc press X and you will be taken back to the main system menu.
5. Run the game you want to backup the saves from, and follow saaviine tutorials if you have trouble.
6. Once you have the backed up "dump" folder in the saavine folder on your computer, you should rename the folder "inject" (maybe make a backup too?)
7. On WiiU - Close the game once the backup is complete and switch to/load up the profile you want to copy to.
8. On Computer - Close saaviine and re-open it in inject mode.
9. With any luck you should just need to re-open the game and then use Saaviine to inject the backup. If it doesn't work, you may need to follow step 4 again (with the profile you want to copy to loaded) and then follow from step 8 onward).

Caveats / Notes

• Profiles have numeric folder names like 80000001 (the default main profile), and if you want to copy from one to another, you may need to change some numbers (ex. rename 80000006 folder to 80000001 before injecting) though I think Saaviine takes care of this for you (it will load to/backup from the profile you have loaded).

• I used FTP anywhere to copy the save files, rather than Saaviine. I had no issues, but Saaviine must be used to load the backup saves... a direct copy lead to WiiU error 150-3032.