Failover clustering event ID 5121 from the application event logs, or the host message NO_DIRECT_IO_DUE_TO_FAILURE.

This error message occurs when the cluster shared volume (CSV) is no longer directly accessible from the cluster node, and I/O access redirects to the storage device that owns the volume. This occurs because only the coordination node can perform actions using VSS backups. During backup operations, the coordination node locks the CSV and requires all non-coordination nodes to redirect I/O.

After the operation has been completed, the coordination node releases the lock on the CSV and I/O is no longer be redirected. If the error message occurs only during VSS backups, there is no failure and this is expected behavior.