O. Format and overwrite FAT partition
then
E. Export FAT partition
*should* lead to a sane userstore exported over USBMS.
Emphasis on *should*

.
(Don't fat-finger this, as there's a couple *very* nasty entries in that menu

).
(
I. *might* be needed if the partition table is utterly screwed, but I have no idea of the potentially harmful side-effects it could have, so, let's table that one until someone wih actual experience with it gets back to you).