Weird SD problem

[hnoto]

Hey everyone! I have a 505, and have 2 different SD cards loaded with different genre's of books. Neither one is very full. I was using one of them for awhile, loading and reading from it. Then the other night, I wanted to read a book on my other card, so switched them out. No problems. Tonight I went to switch back to the first one, and my collections on the SD card appear to have disappeared. The collection name is there, but it says, "no books". The book that is supposed to be in the collection IS on the card, but it is NOT in the collection. And not ALL the collection lists are there.

Any ideas? I've plugged it back in to try to recharge, checked my calibre and the tags are all missing on all of these books on the card.

Any ideas? I can just delete off the card and reload without too much trouble I hope. But my real question is WHY? What did I do and how to avoid this next time?