I just got finished with upgrading my Exchange 2010 Server to SP3 in preparation for next weeks online of Exchange 2013. During the service pack upgrade I encountered a few issues:

  1. After the prerequisite test the upgrade failed due to my antivirus software
  2. Upgrade failed due to the Exchange MMC being open by other users session
  3. Upgrade in a bad state after the two failures above

During the upgrade some of the roles had updated and my install was failing on the mailbox role. Exchange would not continue the upgrade process. To fix this I needed to delete an entry from the registry.

These are the steps I had taken:

  1. Opened regedit (Start > Run)
  2. Browsed the the following key: HKEY_LOCAL_MACHINESOFTWAREMicrosoftExchangeServerv14
  3. Listed here are each role; Hub Transport, Client Access, Mailbox – I looked for the the following: action and watermark entries and deleted them. In my case they only existed under the Mailbox role.
  4. Rebooted Server
  5. Started the Exchange 2010 SP3 Upgrade

This completed with success. After the completion of the upgrade I gave my server a restart and once online and all services running tested via my Outlook Client, Outlook Web Access, and via Mobile Devices to ensure Exchange was running as expected.

I hope this saves you some time if you are attempting this or looking for a solution.