I have tried a few different cards, but no luck. I looked in System Info and I see "Card Reader" in there. I copied/pasted the text it shows under card reader below.
Built in SD Card Reader:
Vendor ID: 0x14e4
Device ID: 0x16bc
Subsystem Vendor ID: 0x14e4
Subsystem ID: 0x0000
Link Width: x1
Link Speed: 2.5 GT/s
Not sure if that tells anyone anything though.
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!