An RDM is a Raw mapping to a SAN LUN. Think of an RDM as a symbolic link from a VMFS volume to a raw LUN. Typically, you would create the C:\ (system) partition as a pure VMDK on a VMFS volume. When creating large volumes, you have the option to either create it as an extra disk on a VMFS volume or use an RDM. A RDM would be interesting when using SAN specific tools in the virtual machine or to quickly migrate from virtual to physical (V2P) when encountering performance issues. vRDM (virtual RDMs) presents the disk as a virtual disk to the VM and you have the possibility to create snapshots. pRDMs (RDMs in physical mode) are typically used in MSCS clusters between ESX hosts or in situations where SAN snapshot capabilities require this.
You can easily migrate between p and vRDMs if you need, let’s say, the snaphot feature of a vRDM. Shut down the host and remove the hard disk/RDM from the virtual machine. Be careful NOT to select “Remove from virtual machine and delete files from disk” when removing the disk. Add the RDM as a new hard disk and select Physical or Virtual as the compatibility mode. After the guest is started , go to disk management (diskmgmt.msc in windows) and rescan to see the disk. Notice that all content is saved!