Even a RDM will show up as a regular disk in the guest, i.e. completely transparent to the OS. When a raw LUN is first attached to a VM, an RDM mapping file (actually two .vmdk files) is created which points to the raw LUN. You may want to check whether this RDM mapping file still exists on the datastore and add it to the VM as an "existing virtual disk". If you still have some older vmware.log files in the VM's folder, it might be worth checking whether they contain hints on where this mapping file is located. You may also try to see whether RVTools (http://www.robware.net/) can help identifying currently unused virtual disks/RDMs.
André