Unable to backup SQL Server from a newly added clustered drive
When adding clustered disk to an instance of SQL Server, there are a few steps that need to be taken.


Present the disk to all possible owners of that disk (nodes)
Bring the disk online from the first node that will own it
Bring the disk into the cluster - it'll now reside in available storage
Move the disk into the resource group for SQL Server
Set the SQL Server resource to DEPEND on that new disk
Double check sys.dm_io_cluster_shared_drives to see that SQL "sees" the disk. If it doesn't, one of the steps above was not successfully completed.


In this case, step 5 was missed which is why it didn't show up in the DMV listed in step 6.