The Flex forum is pretty quiet, but I think this is generic enough where someone can help you here. If you can go into settings and choose about phones and let us know what Android version you are using.
Exactly what I would have recommended if you hadn't beat me to it
The dedicated SD formatter can fix all sorts of card problems, some of which are actually caused by formatting with the generic formatter on a PC. It doesn't do data recovery but it's the definitive tool for reformatting a SD card.