I just got a new 5D Mark III and I am having problems with it communicating with my iMac running OS X Mountain Lion (10.8.2).
I am using brand new Lexar 1000x 32GB cards, and when I connect the camera to my mac it will recognize the photos (in OS X's Image Capture app), but if I try to import them it looks like it is working but it just hangs and after 30-60 seconds I get an "Import Error" message saying the photos couldn't be imported. I have two of these Lexar 1000x cards, and the issue occurs with both cards.
Ironically, the same photos will import to a 6-yr old windows PC (running win7) when the camera is connected to it.
I've also have one of Lexar's older USB 2.0 Dual-Slot Readers, and I cannot read these new 1000x lexar cards in it so I'll be buying a new reader next week. I've used the reader for 3 years with SanDisk cards without issue.
So why is it that I that OS X Mountain Lion is having a problem getting files from my camera (5D3) while Win7 does not? I also have a 7D, and it seems to be able to transfer files from the Lexar 1000x cards just fine (using Image Capture). In fact,I've resorted to using the 7D as a card reader by taking the card our of the 5D3 and putting it into the 7D and importing the photos using Image Capture.
I've tried different cables and different USB ports, but nothing seems to have any effect. I don't think it is the camera as Win7 seems to be able to communicate with it just fine.