[General Questions and Answers] Jeslandor:
Some more information

Hi again. I would like to inform here about some strange stuff I have found while trying to fix this issue for anyone that may be in the same exact situation I am.

First of all is to state again that the instructions given by go_vegan work like a charm in order to make a backup of the SD as well as to access it's content. Just notice again to install f2fs-tools in case your SD is using that filesystem instead of "ext4".

Second is to tell that in my specific case my android rebooted (because of a damaged battery) suddenly and without warning while I was using some app installed on the SD that was doing write operations. So of course the card stopped being recognized by the OS. I first used TWRP to access the card and backup all important files to an USB pendrive. Tried to use the TWRP terminal to do a fsck.f2fs to the SD apparently without success. Next I found this thread and learned how to perform the instructions and what where all about. When the time came to fsck the SD while it showed much more information than the TWRP fsck it seemed unable to repair the filesystem. Lots of errors and the same fail messages were showed over and over each time I did a fsck on the card.

So apparently in my specific case the filesystem was beyond repair. I tried several methods like making a tar with all the files on the SD trying to maintain ownership and permissions and formatting the card manually and then restoring the files also manually. The card and filesystem was recognized by android but as a new SD and the contents installed were not at all.

Then I decided to do another try by letting this time be the android the one formating the card. So to restore the original conditions I run a ddrescue from the backup file to the SD first. This time using ddrescue from windows through cygwin, hoping it would be quicker than using a Virtualbox VM (which was a pain in the ass). And wow, indeed it was much much much much faster. Anyway, and this is where the strange behaviour started. As soon as the ddrescue copy to the SD was done I detached it from my PC and inserted it on the smartphone. I was expecting the storage settings window to do the usual checking and showing the known "damaged" message. But this time it went on and on on the checking phase for quite some time and eventually it just recognized it as good. And it started working as if nothing went wrong. It just showed all the files, all the apps, etc. Working like a charm.

The really weird thing is that it was working apparently well until some time later I decided to shutdown the smartphone and remove the card. From that point on the SD was not recognized anymore no matter what I did. The strangest of all is that when was checking the SD in the PC again I found out that the encrypted f2fs partition was completely gone. It was not there anymore. To be honest no idea what happened or why.

Doing a new ddrescue restore to try again (this time doing a fsck before putting it again in the smartphone).

Anwyay, just wanted to let this info here in case it can be helpful for myself or anyone else in the future. As at least I guess even if the deleted partition behaviour can't be avoided at least it could allow to restore or move apps and apps conf among some other stuff to the real internal storage in case the filesystem just can't be fixed using go_vegan instructions.

Edit: Sorry for such a wall for just a few piece of information. I was never good at summarize stuff.

Edit2: Fixed some typos.

Edit3: After ddrescue restore the SD fsck shows the same kind of fail and error messages originally found but this time is able to fix and repair them. Passing fsck a second time shows everything OK.

Edit3b: Even if the partition seems to be fixed is not recognized by android or even TWRP as adopted storage.

Edit4: After doing some checkings the encripted partition is still there. I thought it was gone because of the way cygwin shows partitions.

Edit5: I guess I can be wrong, but maybe what is happening is that with the previous test the SD was detected as a different card so it was showing two missing SD cards instead of just one. When I did the ddrescue and it finally mounted it and it was working I decided to press the detach/forget/whateveriscalledinenglish button for the other one. Somehow it is possible that even if it was detected as a new different card it was somehow connected to the proper one (that was mounted at that time) and maybe it deleted something needed for the card to be detected as valid.

Edit6: Finally. I was right. When I checked the "/media/misc/vold" folder there was no key file. After doing the fsck let the fix done, check again to see it is all OK I copy the key file to the vold folder and inserted the SD and it was detected again as it was supposed to.

Final edit: I decided to leave the post as I originally wrote it. I didn't have the time to rewrite it.