You mentioned "2 x 256GB SSD RAID1, 2 x 2TB HDD RAID1", so on which disk/volume are you trying to create the RDM? A RDM is a mapping to a previously unused disk/volume, i.e. one that's not already in use as e.g. a VMFS datastore.
What's does ls /vmfs/volumes/disks show? And which exact command did you use to create the RDM?
André