Howdy. Just wanted to pitch in my experience thusfar.
Firstly, I had this exact same problem. Sorry for the late response, but better late than never! I was so genuinely confused because I used this 64GB SD for all my Wii U stuff in the past.
Head on over
here and follow the part of the guide called "[SD Card Mount Failed Error] [KE17]". It's very well written and explains everything you need to do to get the card to work again.
Anyway, I just got my redNAND up and running a while ago, with a formatted sysNAND. I've had past annoyances with my 3DS's linked NANDs, so I just decided to go the unlinking route.
In my current experience, I haven't been able to get a single unsigned thing to install on redNAND. I tried installing a Yoshi's Island GBA VC ticket that I got from
that ISO site, dropped that onto uTikDownloadHelper, and WUPInstaller-AnyTitle spouted an error. Then, I used
this site that was posted earlier in this thread to generate a ticket using a key from
that titlekeys site, dropped it onto uTikDownloadHelper, once again to no avail.