Tom's second suggestion is the most likely reason.
You must never simply unplug or remove a camera or card.
You have to unmount it first.
You do this by ctrl-clicking (or right-clicking) on the item's icon on the desktop and selecting "Eject xxx".
You should then be able to disconnect the device.
However, you may get a message saying that FCP X is still using it.
In this case either leave the device connected or shutdown FCP X and then repeat the eject procedure.
When ejected/unmounted correctly, the item's icon will disappear from the desktop.