Took card out of Canon DSLR, inserted into IMAC SD card slot, and likr you said it's like a hard drive. I tried to troubleshoot by plugging in GoPro camera and it does show it's card's clips, but when I put Canon's card into GoPro it does not show clips, so it must be the Canon.
I watched a tutorial from MacBreak Studio, epidode 187, and they discussed a work around...that you can't see the clips from the Canon when it's plugged in, but if you took the card out and attached through a card reader, then you can see the clips and select ranges to import, which was demonstrated. It did not work for me when I tried it.
I also tried my Sony Handycam SD card in my IMAC card slot and I can select ranges with it, so I am thinking it's the Canon format.