I had the unfortunate experience of someone deleting my Exchange 2010 DAG Computer name from AD this week.
We basically discovered this when the DAG tried to failover for some obscure reason but was unable to do so, due to the cluster actually being down, due to not being able to bring up the computer name.
I wrote a 20 page document on how I fixed this, however due to lack of available time to strip out company specific information, I am going to have to summarise this:
1. Remove all Database copies from all databases ( ie, only leave the active mounted copy )
2. Evict both nodes from the cluster and destroy the cluster using cluster manager
3. Create a new cluster in the Exchange System Manager
4. Using ADSIEDIT, modify the Exchange Server Object and clear the DAG membership field.
5. Return to ESM and add the modified exchange server to the DAG.
6. Repeat step 4 for the remaining DAG Server and proceed to step 5, then skip step 6 🙂
7. Mount the databases ( they go offline for some reason ), and add the second copy or more, depending on your enviroment.
8. Within literally 5 – 10 minutes you will have all your databases in a healthy state, mounted and part of your newly formed DAG Cluster.
For step by step guidance on this, including screenshots, comment and I will send the document to you!
Happy Cluster Recovering!