Hi,
Drone has been sent back for replacement (It had several issues, not just the sd card/camera).
I have done a bit of homework since my last post. You are definitely quite correct about turning off the device before stopping the recording. This apparently confounds the device and totally screws up the data transfer.
It appears that using the canon camera as the intermediary between the drone camera and the laptop was my downfall. (If I had plugged the card straight into the laptop, it would have been OK). The canon camera can only handle a 2GB card and I put in a 16GB that already had files written to it (from the drone camera). Result: Effectively the Canon camera said “'Im not having this” and corrupted the card. Lesson learned.