The SHOW MBR command is a diagnostic, and used to review the
    contents of the disk Master Boot Record for the specified target
    partition or device.

                                   NOTE

       Not immediately useful to users. This command is solely
       intended as a low-level disk diagnostic and debugging tool.

    EFI$CP> show mbr foo:
    Master Boot Record (MBR) not present or not valid; disk is not partitioned
    EFI$CP>