Originally posted by RobG .........
I'm a bit surprised that the camera can't write to the SD card at the same time as doing AF for another shot. .........
There's also the chance that this limitation is intentional for data safety reasons.
If the AF motor were to suddenly draw a lot of current (which can happen if there is friction in the lens or when the lens hits the ends of travel) and cause a power glitch while the camera was writing to the card, the image being written and possibly all the files on the SD card might be lost.
Given that this thread is about faulty batteries and erratic behavior, it would seem wise that camera designers would try to minimize the chance that such erratic behavior damages the data on the card.
Batteries are a bit of a black box (literally!) when it comes to how an individual battery handles variations in current draw. That makes it hard to trust that every battery can deliver a guaranteed level of power at all times to ensure the reliability of critical systems like saving data to the card.