When using WSFC 2008 R2 SP1, the procedure for Extending a Traditional 1×1 2-Node WSFC Cluster to a Shared-Replicated 3-Node Cluster has changed. The WSFC mmc GUI must not be used for adding a node that is hosting a DataKeeper shared volume.
When using WSFC 2008 R2 SP1, additional nodes with shared DataKeeper volumes can be safely added to an existing cluster only with the WSFC Command line tool “cluster /add /node:<standby node name>. VERY IMPORTANT. Please refer to the topic Extending a Traditional 2-Node Cluster to a Shared-Replicated Configuration for more details.