There are serveral reasons to see "buSY" in the viewfinder. Take a closer look at your cameras for other stuff that might light up at the same time, and check what they mean in your instruction manual. For example, . . .
Make sure your battery is fully charged. If the "focus assist" beam is firing when you try to focus, the camera may not take a photo until the flash circuit is fully recharged. When this happens, you will see a small lightning bolt icon when the camera flashes "buSY" at you. This can happen when you are shooting in a relatively dark location, compared to bright sunny day.
The lack of an icon usually means the camera is attempting to write to the memory card. Under this circumstance, the red light should be blinking, which indicates the memory card is being accessed. There are several settings in the camera that can cause it to perform time consuming operations with the memory card. I like to disable any "noise reduction" settings, because you can do the same things in post=processing even better than the camera can.