David: Your friend really doesn't need to power off the device before ejecting the SD card. The card is hot-swappable with no problems.
As to the checks made, as far as I can tell, it is only checking file name and size. If you put a new version with exactly the same name and size, it just uses it. Change the size a little bit, and the books information is completely removed from the database and the book is then added as a new book.
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