I've never encountered this before. Came in from a shoot tonight and was going to download my images off three cards (SanDisk) that I shot with 3 bodies (2 5D's adn a 7D)
I also have MAC and a PC side by side...and two card readers (SanDisk and Lexar)
Neither of the readers...on either machine...would read Any of the three cards. Images still readable on my cameras but could not download them. I finally put one of the cards directly in to my PC that has a slot and was able to pull the images off it.
What...in the world has happened?

