Igorsk hit it spot on... if there are no books then it doesn't show... Mike is right about some devices having size limitations but the last Sony to have any size limit (with SDHC cards) was the 505 and that was fixed with the firmware update (the one that also allowed ePubs).
All current Sony eReaders are perfectly happy with 16GB SDHC cards but just because you can read a card this size doesn't mean you can fill it and use easily. After a few thousand books, the Sony scan gets very long winded... we are talking hours after loading books whilst it rescans.
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