Ciao
it is possible that you are using RDM disks (RDM = Raw device mapping)
In vpshere 6.0 to make a cluster with shared disks it was necessary to use RDMs.
Here are two links with details:
Identify RAW device
https://kb.vmware.com/s/article/1004814
Cluster on VMware vSphere
https://kb.vmware.com/s/article/2147661
Doc on RDM device