Downloading pics

I am using an iMAC OSX. My Canon compact flash cards can no longer can be seen to download pics.
When inserted into my card reader the led flashes as normal but will not show on the desktop. It also freezes the mouse after a short period. The same applies if I try to download from the camera. The card reader & compact flash cards work fine on my PC. This has only started in recent weeks. Would appreciate any help

It might be beneficial to you if you could download the pictures off of your card to your PC, then format the card in-camera, and try mounting the card to OSX afterwards.