I spent about two days on this issue. I could establish a Principal and Mirror pair with the SQL Server 2005, but when an Witness server was added in the mix, the witness could not establish.
I have gotten errors like "Cannot find the server at mirror.xyz.com" and "Alter Database Failed..."
You should go through the article on MSDN http://msdn2.microsoft.com/en-us/library/ms189127.aspx to make sure that you did not botch up any other stuff.
The key to this are the following,
- The mirroring operation is very very sensitive to DNS, if there is anything wrong with it, it won't work.
- The fact that you can "talk" to the mirror and the principal using all aliases is not a sufficient test. Go to each machine (e.g., the mirror and the witness) involved and make sure that all full and DNS alias names can be resolvable to the (identical) IP address and that you can ping the servers with ALL of the known names. If this does not work then you should "hard wire" all the names and addresses in the local machine's C:\Windows\System32\Drivers\Etc\Hosts file.
What Made This Difficult:
In my specific sitaution, there were couple of DNS aliases for all of the servers, and the host name appeared in all different variety of names while confugiring the mirror. As it turns out that having various names is not actually an issue, but the DNS configuration on the TCP/IP control panel was all slightly different causing one server for not being able to find the server name.
Total Time Saved:
You will save at least one day or more of your time by debugging the DNS first.