Seems silly that if you're using an RDM you're limited to the datastore that the machine is running on.
This limitation is most likely due to the SCSI-2 limit explained at http://kb.vmware.com/kb/3371739 which existed in ESXi versions prior to ESXi 5.0
The maximum size of the RDM you can create also depends on whether you want to be able to use snapshots on this RDM (virtual mode RDM) or not (physical/pass-trough RDM) With pass-trough RDMs you can configure the maximum LUN size (2TB minus 512 bytes). With RDM's in virtual mode you need to "reserve" some space for the snapshot metadata and therefore the maximum LUN size must not exceed 2.032GB (see table "Calculating the overhead required by snapshot files" at http://kb.vmware.com/kb/1012384)
André
