I had the same issues and Microsoft store rep;s said it might be 8.1 but they gave me a new one with 8.0 and recommend wait until Microsoft release 8.1 this might be an issues with some of the RT's so my new RT now has no micro sd card issues.
All, I had the same thing happen to me. If you are unable to get to the store to have your Surface replaced or you just don't want to bother, I have a workaround for you. Simply take an ordinary sheet of printer/copier paper and cut a small piece off of it (.25" x 1"). Hold the piece of paper on the back (non connector) side of the SD card when inserting it into your surface. Now you should have access to your SD card. Apparently there are issues with the connectors on some RT units. This is a simple workaround that can save you some headaches. WARNING: Do not cut the paper so short that when you insert it, it is not visible. It may become lodged in the slot and make it unusable. The 1" length gives you the ability to remove it at will. You can trim it down once you've got it working, but make sure to leave at least an 1/8" showing for removal purposes.
I got a 64gb SanDisk microsd card class 10 high speed one. It works fine. I will say it was hard to push it all the way in to lock. I had to use something thin to push it in all the way to lock. But never any issues reading or writing to card.