Have any of them been used in a camera? Do you have a camera that can 'see' them?
• do not try to write to to them -too detrimental to pictures
1. I am assuming that the camera did 'see' the card(s). Is that correct?
I assume that cards used with cameras are the ones you are trying to 'read' to transfer the data/images to the Mac? Most cameras use the Windows "FAT" file format scheme. However, I think the Mac OS has been able to recognize that and alert you to that fact.
2. Can you connect that camera directly to the Mac and download the images?
When I last used real cameras, I was able to do that rather than remove/replace the cards; saving wear and tear on the card/camera contacts (the cable is cheaper to replace). I then reformatted the card in the camera. Also took care of any file system corruption.
Lastly, if you have a spare (empty) SD card, you might try formatting it in the Mac. That should eliminate the Mac's reader from the list of possible problems.