Defective card, card reader? Can you connect the camera to the imac with a cable?
Are you using LR/PS/C1 to look at images on the card or did you try to transfer them to the imac?
I have card from my last shoot in Feb that would not read on my Macbook Pro for some reason (I always transfer to the files but the MBP would not see the DCIM directory) . Luckily I shoot 2 cards so I just used the 2nd card and then verified with another card that the recording slot on the camera was OK.