I didn't see a link to back out of the page for adding a component, for example a hard drive, other than the browser back button of course. Maybe it's there and I'm blind?
You're not blind. :) Once you click from the System Build view to choosing a component, there isn't a backlink to get back to your part list. That's a part of the navigation I couldn't figure out a clean way to present well, at least with the current design. You can, however, click on the System Build button along the top to get back to your part list from any page.