Help: 5D's CF card won't read in PC, but works from camera via USB.

I just had a near heart attack when a CF card from yesterday's shoot didn't want to read in my PC. Although it was my backup camera (for that card anyway), my other two CF cards worked fine, one even from the same camera body.

Here's what is puzzling me:
I have 3 cards, a Sandisk Extreme III 16GB, 8 GB and a 4GB. The 16GB has been in my 40D and is and always has been reading fine from my CF card reader. The 8GB has been in my 5D and did the job without problems. Before this, I always used it in my 40D. Both the 16GB and the 8GB have been formatted on friday before the shoot (quick format in windows 7).

The 4GB card (yes, kick me, I know...) was in the 5D I just bought before the shoot. It was empty when I bought the camera. My test shots were still on it.
I just popped the 16GB and 8GB cards in my camera, and they downloaded perfectly. The 4GB card didn't want to be read, and windows asked me to format it (omgikillyou!).

Fortunately, the images are now downloading from the 5D via USB (sloooowly), so nothing appears to be lost, but my heartbeat is still going crazy.

Can anyone shed some light on this? Could it be a mac formatted card or something that the 5D has no trouble with, but won't read into my pc? Should I just throw it out or only use as an absolute last resort?

You should ALWAYS format the card in camera, not on the computer. I usually do it once the images are copied to the computer and backed up. I have never had a problem with the card reader reading if it had been formatted in the camera.
I would try formatting in the camera and then see if the card reader would read it. If not, I wouldn't use the card if fear of losing the images on the card.

No idea why it happens, but it has happened to me as well. Hopefully, your fix is the same as mine.

Once all the images are on the computer and backed up, go ahead and format the card in the camera and try again. Last time it happened to me (on a 40D), I did the same, and the card has never given me another problem.

hth

Thanks to both, I've never really tried formatting in camera (or came across the option), but I guess it makes sense. Will try that once it's finished importing in LR and backed up in 3 locations.

If it's working in the camera but not in your card reader, I'd find another card reader to test it in. Odds are you have a bent or broken pin in the reader. You might still see the card being detected, and have other hints that' everything's ok, but it ain't necessarily so. Been there.

AC_Photo, I thought the card reader might have a problem, but my 8GB and 16GB cards both work fine in the same reader. I do have another cardreader that I use on the laptop that I can try later.
Right now, all images have been imported, backup is running and then I have 1143 raw images to go through

Same problem a couple of years ago, Downloaded the photos via the camera then formatted in camera and in 4 years never had another problem