All the book details are in one database on the main memory. The entries for the books on the SD card include an id for the card. When you insert a card, it checks if the card has been seen before and then if the books have changed. If the cards is not the same, it assumes that all the books are new. Or at least that's what it did the last time I tested. The details have changed with firmware versions, but the overall function is the same.
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