Lauraphx: I had a card fail last week just as you described and the air temperature outside was 107 degrees when I began the shop. I tried the card in the same external reader I always use, but my pc would not read it. I tried the card in my laptop with its internal card reader, but it also would not read it. I tried opening the card with Word Pad and was able to see data on it, including the names of the files that the DVR assigned (FAMH01~1ASF) etc. I have not been able to read the files, and the file shows a size of only 32kb.
I put the card back into the DVR and tried to do a test recording, but the DVR would not read the card. I put another card in the DVR, and it worked perfectly, which indicated the problem was the card and not the DVR. The DVR was working before I left home and again when I started it about a minute before driving into the apartment community. I theorized that something, possibly a static electricity discharge, may have fried the card moments after it began recording.
I never had this particular issue before. I thought maybe that the combination of the dry heat and something else in the environment may have combined to cause a static electricity discharge at the wrong moment. The small file size leads me to think the DVR stopped reading the card seconds after beginning the recording. I gave up on it and decided time was better spent doing more shops. I just will not be using that card anymore.