Very helpful, thank you Radek! We are on 7 mode. I had read the installation and administration guide but it didn't seem to 100% speak to this issue. The updated Best Practices Guide you linked to does have this item:
For clustered instances of SQL Server:
The SnapInfo LUN must be a cluster disk resource in the same cluster group as the SQL Server instance being backed up by SnapManager.
Place user databases onto shared LUNs that are physical disk cluster resources assigned to the cluster group associated with the SQL Server instance.
Is this what you are referring to?