- This is a good article talking about using either VMDK or RDM for Exchange storage.
On the other hand, as far as the VM backup:
Backup software such as vRanger can only back up disks that VMware can snapshot. This includes Thick provisioned VMDK, Thin provisioned VMDK, or Virtual mode Raw Device Mapping.
It can not back up VMDK files that are marked as Independent, Physical Raw Device mappings, or disks presented via an iSCSI initiator from within the Guest OS.