PROBLEM:
You're mSD is bunked.
You're mSD is perma-locked. Go ahead, check it in Diskpart. You can't undo it either. This is a failsafe feature of mSD cards.
I had to buy a new one (same error/issue). Couldn't even get it to reformat. Which means it locked down after exceeding the write limit. So I now have a permanent physical backup of my old 128GB mSD. Funny thing is, you can READ the mSD, correct? Yeah... but you can't WRITE to it. Hence the FBI and 3DS error prompts when trying to install/update. It can read your files. It just can't write/delete them. You can try to reformat, but since you can't WRITE you just end up wasting time. Same goes for writing files to mSD. It -LOOKS- like it worked, but take it out, pop it back in to PC, and guess what? That file isn't there.
So what's happening here?
After a while the mSD starts to fail and locks itself from writing which you can't fix. I tried. Even used DiskPart, etc.. However, this does give you time to copy/backup the files before it really does die on you.
NOTE: I did not try using LINUX Terminal yet. There may be hope... but I just didn't have time to boot into ULBD to see if I could get it to reformat/unlock.
However, I have seen this happen before with phones/devices and it has nothing to do with encryption or unit. It's the mSD going bad. So it perma-locks itself, giving the user time to backup files before a complete meltdown.