You could try the second link I posted you, it covers specifically adding a second hard drive rather than replacing.
[url]http://fogproject.org/forum/threads/how-to-add-a-second-hard-drive-to-fog-server.4383/[/url]
I only posted this information so you could give it a shot. I personally haven’t ventured into increasing my drive space, or adding another drive.
You alternatively could use the Web GUI after you mount and format the new drive to add a storage node. It all depends on how you want to set it up.
I posted these here in case I misunderstood what you were trying to convey (which I did 🙂 )