Stencil and depth buffer questions

What I would like to know is this: is there only one Z-buffer and stencil buffer that all the color buffers use, or does each of the color buffers (FRONT, BACK, LEFT, RIGHT…) have one attached to themselves? For example if I draw something to the back buffer and then switch to the auxilary nummer one using glDrawBuffer(GL_AUX1) and draw something there, will my Z-buffer data from the back buffer then be lost? Also, the red book says, that it is possible to have more than one color buffer active at one time for drawing. How can I be sure that I don’t draw in a certain buffer? Is it enough just to specify another buffer with glDrawBuffer() or do I have to disable it in some specific way?