Comment 7 for bug 743303

You are unmounting the card reader when you only need to eject the card from the reader which leaves the card reader mounted and able to read the next card inserted into it. There should be a second option, and in Linux Mint 12 it is 'eject' which is the correct option to enable the card to be safely removed.
A USB memory stick is unmounted before removal, but SDxx memory cards are ejected, leaving the card reader mounted and ready to read the next card inserted.