Diagnosing corrupted files

I lost the photos I took with my 6D over the weekend and am trying to identify where things went wrong. I could view the images in-camera, but Lightroom didn't recognize the files as images, and when I put the card back into the camera it rejected them as not being displayable. My expectation is that it's the card going bad (a Sandisk 8GB Extreme III I've had for years) but in the interest of mining the communal knowledge well, here is the story of my absolute disregard for best practices:

1. I removed the card from the camera while it was powered on, but asleep.

2. My computer (a Mac) was sleeping and had another SD card in my USB card reader. I removed that card without ejecting it first - I received no error message when I did this and I expect that the card was already ejected from a previous session, but it's possible the old card was still mounted.

3. When I plugged the new card into the card reader, I received an error message that a SD utility had crashed. I think this was related to the Canon photo utility, a piece of software I never use but that automatically opens whenever I mount a SD card. (I really should disable this.)

After following this series of errors, Lightroom would not recognize the images on the card, and it saw only 35 files when I'm certain I took closer to 100 photos on the card. Moving the files from the card to my hard drive didn't resolve it, and as I said earlier, the camera wouldn't display them, either.

If the card isn't bad, the possible culprits are: (1) removing the card with the camera on scrambled it (unlikely, as I've done this many times before); (2) mounting the card when another card had been mounted in the card reader somehow scrambled it; or (3) the random SD card utility somehow destroyed my files.

Again, I expect it's the card, and now I've experienced why pros insist upon the dual-card setup in the 5d series . . .

First, there are a variety of free data recovery programs out there. Recuva and ZAR are two good ones. They should be able to recover your images safely. Use either or both as you wish.
I would toss the memory card. I'm not sure from the description that you gave that you caused the problem, but all storage media wears out. HD's are not reliable after about five years. You have gotten your money's worth out of this card. Don't take a chance on it. Get a new one.
This probably isn't necessary, but I assume you are formatting your cards in your camera each time, not in your computer. Not doing that can cause the problems you have described also.

Thanks for the advice. I've downloaded a utility - I'll see if it can recover the files. I'll report back here with the details and results if it happens to work. Finding information on this process is remarkably counter-intuitive for Mac users who don't get under the hood very often.

I agree that it's time to toss the card. I have plenty of others.

I do always format in-camera. The first shots of the weekend were by my wife, who doesn't know how to use the camera at all, and I specifically recall formatting the card in-camera before handing it to her.

I would not toss the card because this sounds like a simple file system error, not a card hardware failure.

I has a similar problem except that is was caused by using a new card reader that basically chewed up the file allocation table, rendering the card unreadable. I used recovery software and was able to retrieve all the files (because the actual card memory was fine, the system just couldn't see the files due to the damaged file list). Once I put it back in my camera and reformatted it (post recovery) the card was fine and has been fine ever since.

The problem could have been the unsafe removal of the card from your camera or the computer not recognizing the swapped memory card and trying to work on the filesystem using a stale file allocation table. Both the camera and the card have measures to protect against these problems, but the fact they were both sleeping and possibly didn't wake up in time to handle the error might have caused the issue.

In general, once you open the memory compartment door on your camera you should wait 5 seconds before removing the card. This gives the camera enough time to detect that the door was opened, then update and close the allocation table on the card.

Of course you should always eject a card before removing it from your computer. But normally this would result in corrupt files on the card you removed, not on the one you inserted afterwards. And in any event the mount process should recognize the card change and re-read the allocation table, but there could have been some other transient error there.

I would perform a full reformat of the card (after recovering your files, of course) and then spend some time shooting test images and transferring them to the computer before discarding a potentially good memory card. Then again, 8GB cards are cheap so it might not even be worth your time for testing.