I had the same issue. What I figured out was that I mounted the ISO from within vCenter and when it went to install SSO it disconnected to install the new services. The easy way around this is to use your vSphere client to connect directly to the host that your vCenter server runs on. That way it will get through the install with out disconnecting your drive.
Other info:
I was upgrading from 5.0 - 5.1, so results may vary.
Good luck!