If you have shared storage showing available in Failover Cluster Manager but SQL Server Setup throws an error indicating there is no shared storage available, do the following to fix the problem:
Add the shared disks to another resource group (such as MSDTC).
Then perform the Remove From (Resource Group Name) action.
This returns the disk to Available Storage, and SQL Server Setup can now detect the shared volumes.
This was seen with SQL Server 2008 R2.