Well, I actually did try that, and that came up nada as well. So I did what I probably should've done in the first place: I backed up all the files, formatted the card (fat32), and then put the files back.
Problem gone.
Not technically solved, of course, as it seems I'll never know what'd happened, but at least it does write correctly now. Thanks so much for the timely responses tho! I am glad to see these forums still kicking, and I'm sure I'll need help with some other things in the near future (my list is reaching critical mass). Good health, and happy coding! (Or is that bashing? ...hacking? Or is that a misnomer? ...You get me.)