I suspect that it's because it is an SD card slot, which is incapable of reading the newer SDHC cards.
Current MacBook Pros have SDXC card slots, which will read SD, SDHC and SDXC cards.
Had problems reading SD cards after a Mountain Lion update. Try this XQuartz solution that fixed this for me. I'm using a 13" MBP mid-2012. All you have to do is install this programme and re-insert your SD card.
XQuartz no longer comes with new Macbooks, only X11.
See this support page from Apple on where you can download XQuartz.
You can download Xquartz directly from here:
This should be the most up to date version. If not check for updates once you have downloaded it.
Simply re-insert your SD card and voila!