<p>For long exposures, the default behavior of the camera is to take a second exposure, of equal length, without the shutter open. It uses that image to subtract out noise from the previous photo. This is called a dark frame exposure and the process is called dark frame subtraction.</p>
<p>So, if you have a shutter set for a 30 second exposure, the camera will be exposing the image for 30 seconds, and then perform the dark frame exposure for another 30 seconds. So your camera will not respond (unless you turn it off) for a total of 60 second.</p>
<p>You can turn this option off in the CFn menus. </p>