This is what happens when your server is using WSUS Windows Server Update Services and has 40gb of updates it is trying to do to the workstations. It sits there with 40gb of space for no apparent reason and causes exchange to stop working when it drops below a certain threshold (in this case, 5gb left). How do you get windows to stop shooting itself in the foot?
In some cases you don’t need updates to be performed this way so change the following settings:
Screenshot of WSUS: