Took one flash and it worked then subsequent flashes said "busy". I tried new battery and reinserted memory card but still says "busy". Using Canon 20D (in P and Tv modes).
Do you mean the camera is saying busy? Thats related to writing to card, are you getting new images on the card without flash. I guess my gut here says this is not a flash issue, stupid question is the card old? Is the card full? This is how my card acts when full. My next steps would be to format the card and see if it is still having write issues, then try a new CF card as yours may be starting to die if its old, you really should have more than one card. I would not worry about getting the largest or fastest cards, they should be backwards compatible but definitely are overkill and the speed wont be able to be used. https://support.usa.canon.com/kb/index?page=content&id=ART160782
Does it take photos when there is not flash attached? What flash do you have? Have you tried manual mode on the camera? Your need to get a focus lock for the shutter to trip and the flash to fire, how much available light is there? It does not sound like your issue but I'm clarifying anyway, as I mentioned the busy sounds like a card write issue to me. Either way very little detail on the exact gear and exact settings, and lighting situation so I am making some assumptions.
