Ran into this problem today, but the answers weren't entirely clear.
By the OP's description, he dragged the SD card icon off the Sidebar in the Finder (in the older OS, this was animated with a puff of smoke and occurred with the Dock also). This removes from the Sidebar the shortcut to access SD Cards when they're inserted.
It can be activated again through Finder > Preferences > Sidebar and checking the box next to External Disks (you may notice that the box has a minus in it, indicating the removal of the SD card shortcut).
Either way, the card can still be accessed via it's icon on the Desktop if your Finder preferences are set to do so (General tab).