I also got this Busy error message when trying to shoot. I was shooting rapidly with either my internal flash or my PocketWizard. I don't remember which. Later, at another shoot, the whole camera locked up with just an "Error" message. I removed the battery (which is a generic no-name battery) and put in an official Canon battery. I haven't had any problems since. I too have a large 32GB 600x CF card.
Not Telling
