While the absolute min. HD is 32GB wether you have 8GB RAM 16, or 96, each requires a different size of Hard Drive partition for the OS. ( http://technet.microsoft.com/en-us/l.../jj134246.aspx
The installation routine has its own algorithm to evaluate the current situation, as a result manipulating part of the system often does not work.
I assume that the server is running 24/7 for a while on the current drive, it might be a good idea to get a New less "tired drive" too.
Try to clone the current drive to a larger drive, then try to upgrade the installation that is on the larger drive.
Take out the old drive and leave it alone as a backup until the new installation works.