You may want to check the card to make sure it hasn't been reformated in an incompatible format. The following article may be helpful:
Can I use OS X to see the specifications for the interface hardware and media inserted in the SD card slot?
Yes. The System Profiler should provide information on the interface hardware and the media you inserted into the slot. To access the System Profiler, choose About This Mac from the Apple () menu. Click More Info, then select USB in the Hardware section. A list of USB devices should appear. Select Internal Memory Card Reader to access information about the interface hardware and the media inserted into the SD card slot.