Having a
'spare' is a great idea
Flash (internal and External storage ) can go bad
with little notice .
It happened to me while loading a new book onto my SD CARD.
One minute it is loading and updating the Calibre metadata, the next: ERROR
Toast . No SD Card device (I have many) could touch it.
Lost all my reading progress locations (I have many books 'going' as the mood fits) even though I did have the books safe in Calibre (also backed up).
Moral:
Have backup of 'hardware' also