There are some PC tools to recover data from corrupted SD cards. I’ve successfully used photorec (available for Linux, Windows and Apple computers) a number of times to save data off otherwise unreadable cards.
No telling how well that will work if part of the card is set as internal phone storage, but these kinds of data recovery tools are pretty good at pulling data out of broken filesystems, so it’s worth a shot. Pull the data out of the card and then reformat it to be usable again, then transfer the data back to the card.