raw means any "whole" block device, meaning to snapshot, clone, no shared SR case, etc. Doesn't matter what's underneath. I would say the first use case is passing a whole drive to a VM (eg FreeNAS)

Thanks. I guessed that I needed to put the full path on FreeNAS as there is not masking when expose as NSF by FreeNAS. So I put the full path as "/mnt/MybooksVolume/VM_ISOs" and that seems to have fixed the problem.

Thanks you Oliver!

It's an admin privilege.