You add and remove datastores to and from an existing datastore cluster by dragging them in the vSphere Client inventory.
You can add to a datastore cluster any datastore that is mounted on a host in the vSphere Client inventory, with the following exceptions:
All hosts attached to the datastore must be ESXi 5.0 and later.
The datastore cannot be in more than one datacenter in the same instance of the vSphere Client.
When you remove a datastore from a datastore cluster, the datastore remains in the vSphere Client inventory and is not unmounted from the host.