Hello, I have 2 Sandisk SDHC 4GB cards from which I cannot delete files or format the card. This problem is present if the cards are in a camera, a card reader or another computer. I have taped the lock switch, tried defragging (appears to do it but when analysed there is no change). If I transfer files to the card they can be read from the card but if the card is removed and then inserted again the new files are missing. The camera will not take photos with the affected cards. The problem exists on 3 separate pieces of hardware.