Without further details it's hard to say why the mapping file wasn't migrated. However, The mapping file is only a pointer to the LUN and you can either connect the VM to the existing mapping .vmdk file and/or - if you prefer - remove the RDM from the VM (if attached) by selecting "Remove from disk" rather than "Disconnect from VM". This will only delete the mapping file and you can create a new one on the target datastore by adding the LUN as an RDM again. The data on the raw LUN will not be harmed doing this.
André
Discussion moved from Community Website Help to vMotion & Resource Management