Strange CF card behavior – corrupted? (not panicking)

I have a Sandisk Extreme 16Gb card that when I put into any of my cameras none will turn on. I should say that all of my cameras have Magic Lantern on them, however, normally if I put a blank card in, they still operate just fine, they just don't load ML (and I have a completely blank Lexar that I have tested with).

I had been using it on my 5DIII fairly regularly before I had this problem. I re-formatted it on my PC and it seemed to work again, but after using it, taking it out to transfer images and putting it back it the camera I had the same issue. I would like to try formatting in-camera of course, but have not been able to since this issue started.

The strange (and nice) thing is that I never lost any photos because I can read and write to the card through a card reader on my PC. I tried formatting it (low level this time) on my PC, but no change.

I’ve already ordered a replacement, but before I toss this card I was wondering if anyone had any ideas on what might be up with it. It just seems a strange behavior, I mean even if the card door is left open you get the red light blinking, but with this, nothing.

Take a look at both the card reader and the card slot in the camera and see if any pins are bent/missing. Also look at the card (and the working Lexar card) for a pin stuck in one of the holes. If you've, through whatever accidental action, forced a card into either your card reader or camera (more likely the card reader, Canon's card slots are pretty well to spec), it's possible to bend/break a pin.