Busy signal while taking a picture

Have you been taking shots in continuous mode (burst)?  The camera has a built in buffer of fast memory where photos are stored before being written to the slower CF card.  Once the memory gets full the camera will not take another picture unless some of the images complete transfer to the CF card. You will notice a number inside your viewfinder (bottom left) the number goes down when you take a picture that is how much space is left in the buffer when it hits 0 the buffer is full it will go up as the buffer clears.  Note that RAW takes up more space and fills the buffer quickly.
If you get the "Busy" message even after just 1 shot then there is something wrong with either your CF card or your camera and you should take try another card or it to canon for service.