Our plan is to have two internal cards, with a mirroring scheme, and an external slot for the user.
The interrupt seems to reliably signal the removal of the card which I use to set a flag that is checked before read or write.
That’s what I’m doing, now; see sd_card.c.
It goes further than FF_Invalidate(), calling FF_Unmount(), and basically resetting to uninitialized state, since a different card might be inserted next.