Thank you all for your replies. They have brought up many interesting possibilities.
I went to Africa on this trip taking 17 4GB and 8GB cards. I had expected that they would last the entire tip and I had a NEXTO 500GB has a backup. Unfortunately, I filled up the cards after the first week and then started reusing the cards (yes, I know I lost my redundancy).
All of the scenarios presented are possible because of the above. Does anyone know if Opanda or any other software records a serial number or any other ID of the original card that it gets recorded on? That would make it easy to see if a card has went bad.
Thanks to everyone for their help and suggestions.