In order to have a clear picture of your issue let me ask you what RAID controller is installed on your server? Or are you using any of the embedded software RAID solutions (ESRT2, RSTe)?
In the meantime consider that Vmware* ESXi does not support any kind of software RAID solutions and will detect the HDDs as AHCI individual devices.
Furthermore ESXi 6.5 does not show up under the tested OS for this board, you can check it on the following document:
Even thou, the implementation of an actual hardware RAID controller like the ones listed on http://www.intelraid.com/ http://www.intelraid.com/ should allow you to install ESXi 6.5 on a RAID volume and be detected by the VMs.
Will be looking forward for your comments.