In a previous blog post I outlined a failure behavior of the file share witness resource on Windows 2008 clusters.
Ultimately there could exist a condition where the cluster enters a lost quorum state due to the File Share Witness resource being in a FAILED status even though the witness directory was fully available.
In Windows 2008 R2 a design change was made. When the File Share Witness host server becomes unavailable, the File Share Witness resource will still fail in cluster and cause the Cluster Core Resources to move between nodes. In this case assuming the File Share Witness host server is still not available, the resource remains in a failed state. If it becomes necessary to utilize the File Share Witness to maintain Quorum, and the witness resource is in a failed state, cluster will attempt to online the witness resource. If the online is successful the witness share is alive and accessible – quorum is maintained. If the online is not successful, the witness share is not alive and accessible – a lost quorum condition is encountered.
A hotfix has been released (KB978790) that back ports this change from Windows 2008 R2 to Windows 2008.
You can download this hotfix by navigating to http://support.microsoft.com and requesting the knowledge base article 978790. There is a link to request and download this fix in the upper left hand corner. (Note: When downloading the fix the package is marked for the product Windows Vista, but this is actually for Windows 2008).
This hotfix is applicable to Exchange 2007 and Exchange 2010 when utilized with Windows 2008.