A few people are reporting that a Class 10 SanDisk SD Card won't be reliably recognized in a 2011 MacBook Pro.
What does seem to work as a temporary workaround is restarting the Mac with the card already inserted. But even then it's not recognized if you remove the card and then reinsert it or another similar card.
Someone tried a card in MacBooks at an Apple store and:
Apparently Apple and SanDisk are aware of the problem but if you use SD Cards for production or have them sent to you for editing it's a good idea to be aware of this issue. I've not seen it as I've been using an external USB card reader. Hopefully it'll be addressed soon with a software update.