The Migrate a Cluster Wizard could not determine the available drive letters on your cluster because some nodes are down. ...

The Migrate a Cluster Wizard could not determine the available drive letters on your cluster because some nodes are down.  Migration will complete, but it will not check if the drive letter is available on all nodes in the target cluster.  If one of the down cluster nodes already has a drive assigned this letter, the new cluster disk will override that assignment.  If you are sure that these nodes do not have the drive letter assigned to another disk, you may continue safely.  Otherwise, to avoid this potential problem you can cancel this wizard, start all nodes in the target cluster, and run this wizard again.  Do not select the 'Ignore Down Nodes' check box.