I just started experimenting with video on my 5DmkII and it’s been frustrating. The camera repeatedly locks up during recording. It says the buffer is full, flashes a “Busy” message on the status LCD, and won’t power down. The only solution is to remove the battery and lose the video that was recording. Not ideal.
It took me a while but I finally came across this blog post that describes the problem and solution.
Basically the problem is a result of using 8Gb Sandisk Extreme III Compact Flash cards. I have 6 of these and they are the only cards I use in my Canon because they are so reliable (ha!). So it’s off to the store to get another brand of CF card and see if my video problems go away.