I've hot swapped a card while FBReader had an e-book open from the RS-MMC card. While I had the card out, I actually replaced the e-book text with a new version and then popped the card back in. It didn't seem to have any ill effects, but I didn't play around with it for too long. I switched back to the library index view, then tapped the same book, and the new version came up with my latest changes. No crashes. Like Hedgecore says, though, it's probably different with something like an MP3 file.