Can anyone please point the requirements to allow protection if an RDM based machine.
Is there a particular SCSI controller type I have to select for this to work?
There are some limitations when protecting to/recovering RDM volumes.
One limitation in particular to your question, the protected VM cannot be using the BusLogic SCSI controller.
The online documentation outlines the remaining points of consideration, under the “Protecting Virtual Machines from a vCenter Server” section. Hope this helps!