You may find yourself in a scenario where your SBS 2008 server has died and you’ve no backups available, however you do have a second non-SBS domain controller that’s still operational which contains all of your domain information. Follow steps in this post to rejoin the SBS 2008 server back into the existing domain so you don’t have to recreate all of your AD objects and rejoin your client machines. Preparation and Clean Up Steps: 1. Change primary DNS server IP on TCP/IP properties of NIC of second DC to point to it (e.g. 127.0.0.1) 2. Ensure second DC is a global catalog server. Open AD Sites and Services and go to properties of NTDS settings of second DC and check global catalog box if it wasn’t checked 3. Verify which FSMO Roles were held by SBS 2008 by running “NETDOM QUERY FSMO” from an elevated CMD prompt 4. Seize all FSMO roles the SBS 2008 Server held to second domain controller. From an administrative command prompt open ntdsutil utility by typing NTDSUTIL and pressing ENTER.
More info: SBS blog