I was recently building a SQL Server failover cluster and had an issue where I had two SAN-connected drives to be connected to two separate nodes, but only one drive would appear as available in the Windows Failover Cluster Manager to add to the Disk Storage.
The solution turns out to be a series of Powershell commands.
First, find the ID number of the disk that you're interested in:
Then you clear the cluster reservation for that disk number:
Clear-ClusterDiskReservation -Disk 2
Go back into the Failover Cluster Manager, and the disk should be available to add to the cluster.