Setup: I have an existing 2 server exchange organization (2003 SP2 FE & Mailbox). I built/added 2 new Exchange 2010 SP1 servers (CAS/HT/Mailbox). All proper prerequisites were followed, including all AD related prerequisites. All went perfectly well. These were clean installs and nothing was changed/done with the Ex2010 configuration, with the exception of creating a CAS Array. After attempting to install Ex2010 SP2 on one of the servers, the upgrade failed and I now get a message when attempting to rerun the upgrade that the installation path is unavailable in the registry and that only disaster recovery mode is available. Exchange 2010 does not show up in Add/Remove Programs. The EMC or EM Shell will not launch.
As of now the Exchange 2003 environment continues to work normal and as far as I know the other Ex2010 server is still alright.
I need an expert to tell me what I need to do to correct this issue. Research is indicating that its going to take a clean reinstall of Server 2008 R2 and a reinstall of Exchange. I need specific steps with details because of the integration of AD and the existing Ex2003 org that it was introduced into. I do not want to mess up AD or the existing Ex2003 environment.