I would try to isolate the conditions to reproduce the problem.
If the buffer is full and has delays in transfering to the card it will not shut off immediately. So try removing the largest card and fill the buffer before turning it off. Check how long the memory activity light flashes. Swap cards, remove the smaller one and use only the larger and try the same test and see if there is a similar time to clear the buffer. I have never tried a card that large, too risky if the inevitable card failure occurs before the card has been transferred to a hard disk. Smaller cards, changed more frequently is a safety and lower cost feature.
Stan
St Petersburg Russia

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