I just got a GoPro Hero3 a couple days ago and ordered a 64GB Class 10 card with it.
The card seemed to behave fine in the camera, but I couldn't access it from my desktop.
Turns out the 64GB cards come formatted to exFAT.
XP requires update KB955704 to be able to handle exFAT drives.
It was a simple fix, but took a little searching to find the answer.
Anyway, thought it might help someone else out down the road.
Now, to have some fun with the GoPro.