Android 2.1 - MyTouch 3G Slide - Preparing SD Card mal-function - Formatted SD Card after taking out and putting back in :(
Android has to mount your SD card; that's all that "Preparing SD Card" does.  If that doesn't happen, then the device can't use the SD card.  Hopefully the format was just a fluke.  You should put test files on it (unimportant / already backed up) and see if it happens again.