I use a Canon EOS 40d DSLR for most of my photography. From time to time I get Err 99. I read a lot about people postulating on what this was. I've an interesting observation, I think it might be related to battery voltage under load.

Particularly on my older batteries I will start getting a run of Err 99 when it is cold, shooting long exposures or using the inbuilt flash. If I replace the battery without changing anything else the problem is gone and I can shoot to my hearts content.

I suspect that when the shot is taken the power used causes a voltage drop. The software senses this just prior to writing to the flash memory and refuses to write (so it won't get corrupted) and displays Err 99.

I have not seen this posted anywhere so I thought I'd put it on my site for the benefit of others.