In reply to Climbing Pieman:
> I've noticed now the darker nights are here, that longer exposures take almost exactly the same length of time to write to my SD card, ie 1 min exposure, 1 min to write; 2 mins - 2 mins; and last night a 4.5 mins exposure took 4.5 mins.
My guess is that you have long exposure noise reduction turned on.
The extra period of time is not 'write time', but the camera taking a second exposure, but with the shutter closed.
The camera will then use the noise-only exposure to evalute how best to deal with the noise in your original image - It then writes the image to the card.