I'm struggling to understand how to do differencing disks in VirtualBox v3.1.0. I've created a Windows 2008 Server, but now I want to use that as a base image for a number of other servers. The help file has a description of what differencing disks are, but I can't find where it actually tells you how to do it. In the Storage dialog for a server I found the Differencing Disks checkbox: but when I check it, I'd expect it to then ask which image should be the parent so I could select my base image.
Any pointers you can offer would be greatly appreciated!