Time Left: Look at your recorded shows, or record one if you haven't already done so.
The percent used is shown at the top. It's not in terms of hours, it's just a percentage since HD shows eat up more disk space than non-HD.
While in the guide mode, push the PLAY icon key on the remote, it advances one day. There may be other ways to do it.
I'm trying to find information on adding an external hard disk. I got a call from COX telling me about exciting new features, adding my own drive to expand the storage was one of the features, the message was brief, and I can't find anything more on the cox website about it.