Since the camera can see the images this would tend to point to an issue with the card reader on the Mac. I would try reading the card with another computer to see if you can access the files.
I had this problem once with my MacBook Air. I connected the card to my wife’s Windows computer using an SD reader and I could copy all of the files to her hard drive.
I put the card back in my MacBook and reinitialized it and the card now seems to work just fine.
Personally, when I have a memory card malfunction like this I take is a sign that I should replace the card. Once a card malfunctions it is not unlikely that it will do so again and I don’t want to trust my data to an unreliable storage device.
If you do find yourself with an inaccessible memory card you can try a data recovery program like PhotoRec. Although bot as slick and as intuitive as some of the commercial data recovery applications, it is free and they offer step by step instructions for using the program on their site.
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