I never contact Co-pilot support as I figured out from my own research. It is known documented issue caused by VAAI ATS heartbeat and disabling it on hypervisor have resolve issue for me. You can look at detail on VMware KB at  kb.vmware.com/.../search.do 
Hi, We are using SC4020 as a iSCSI storage for our Virtual Environment with vSphere 6 for a while but now we are seeing issue of "Lost access to Volume..." throughout environment which brought me to question on VAAI and SC4202 compatibility with ESXi 6  Do Dell/Compellent recommended to have VAAI enable or disable?   Thanks
Compellent SC420 and ESXI6