I don't think it is the Apps. File A (and B, c, etc) were created on the SD card and were accessible for update. My internal storage needed space so I accepted the invitation to transfer files from Internal to the SD card - these were actually photos I had taken. Once I had completed the transfer of photo data the files created on the SD card (File A, B, c etc) became 'read-only'. These files were always on the SD card, have never moved, but the access rights have changed following the transfer of other files from internal storage, I think it has done something to the SD card root directory making all SD card files read-only but I cant find a way of recovering the original access rights.