I had this happen to me too! I'm also on a Mac.
What I did to fix the problem is I ran Disk Utility.
It was my first time using Disk Utility but it was pretty easy. I first clicked on Verify and it told me there were errors. And then I clicked on Repair and voila! All fixed. :D
Let me know if you need step by step instructions. OH and ALL MY FILES WERE STILL THERE. :D
So DO NOT FORMAT YOUR CARD -- just repair it. :D