I don't think it's the cards fault though. I tried other cards in the same reader and all of them show the same message?
Probably the format rather than "the card". Camera's are sometimes prone to it, I think extracting the card and putting that in a computer is a minority of users, hence camera manufacturers sometime stray from ensuring their formatting works with all OS's directly.
When using the USB cable the camera is acting as the reader hence the card format isn't seen by the computer OS directly, it is seen as a external USB drive which seems more robust across OS's (and/or the camera manufacturers put more effort into making it more robust
)
Sometimes camera models can behave differently in this regard but I've found they seem to be consistent, if the model you have formatted isn't readable in your Mac with your OS version, it never will be unless the OS changes (the camera format will remain the same as camera's generally can't/don't have their firmware updated in order to change the formatting of the card....)
Just thought, always format in the camera, if it is then readable by the Mac don't use the Mac to delete images, reformat in the camera. You might find it is the deletion in the Mac since camera format that can cause it. Then ONLY use the camera to format the cards, not your Mac....
