Some environments maintain copies of the production LUNs as a backup, by replicating them. These replicas are exact copies of the LUNs that were already presented to the ESXi hosts. If for any reason a replicated LUN is presented to an ESXi host, then the host will not mount the VMFS volume on the LUN. This is a precaution to prevent data corruption.
You can, however, force mount such a volume, but you need to make sure that the original LUN is unpresented.
The act of mounting a snapshot volume can be done in two ways:
Using the vSphere Web Client's New Datastore wizard
Using the ESXi CLI
The following procedure will guide you through the steps required to mount a VMFS volume on a replica LUN or a LUN detected as a snapshot:
Use the vSphere Web Client to connect to vCenter Server.
Navigate to Home | Storage.
With the data center object selected, navigate to Related Objects | Datastores and click on the New Datastore icon...