This article explains why we do not support backups of Clustered Shared Volumes.
High Availability Cluster : A High Availability Cluster is a group of 2 or more bare metal servers which are used to host virtual machines.
Clustered Shared Volume : A CSV is a disk or pool of disks which is accessible by each node as if it were a logical disk on the system.
We are unable to directly support the CSV backup processes due to the manner in which the STCVSM filter driver is read within Hyper-V. The filter driver ends up picking up an altitude that forces Hyper-V into redirected access mode when running backups. This is a ultimately a short coming of the Hyper V interacts with it's VM's. In VSphere for example, it does not pick up specific information about driver versions on the VM's, so the altitude is not as problematic.
Storagecraft's Documentation :