The title says it all.
I created the disk and now I would like to increase its size. I know I could add a new one, but I don't want to.
The guest os in question is Ubuntu 11.04
There is a very good blog entry on this at http://trivialproof.blogspot.com/2011/01/resizing-virtualbox-virtual-hard-disk.html
Basic instructions, to resize to 30G:
There's a section in the VirtualBox Manual which tells you how to resize a VM Hard Disk using a cmd-line tool. Would this be any good?