Have you tried converting the RDM LUN directly into a VHDX without the middle step? See example 4.
PS C:\Users\administrator.SMIS-RTP1> get-help -examples ConvertTo-NcVhdx
NAME
ConvertTo-NaVhdx
SYNOPSIS
Convert a VMDK file, a fixed VHD file, or a Data ONTAP LUN into a VHDX file.
-------------- Example 1 --------------
C:\PS>ConvertTo-NaVhdx -SourceVhd F:\VHD\win2k8r2.vhd -DestinationVhdx F:\VHDX\win2k8r2.vhdx
Convert a VHD file to a VHDX file.
Mode LastWriteTime Length Name
---- ------------- ------ ----
-a--- 8/16/2012 3:05 PM 26847739904 win2k8r2.vhdx
-------------- Example 2 --------------
C:\PS>ConvertTo-NaVhdx -SourceVMDK X:\VMDK\Monolithic\win2k8r2.vmdk -DestinationVhdx X:\VHDX\win2k8r2.vhdx
Convert a VMDK file in a CIFS share into a VHDX file.
Mode LastWriteTime Length Name
---- ------------- ------ ----
-a--- 8/16/2012 3:00 PM 26847739904 win2k8r2.vhdx
-------------- Example 3 --------------
C:\PS>ConvertTo-NaVhdx -SourceVMDK /vol/vol2/cifs/VMDK/Monolithic/win2k8r2.vmdk -DestinationVhdx
/vol/vol2/cifs/VHDX/win2k8r2.vhdx
Performs the same operation as Example 2, except it uses the direct path on the controller to the file. This is
useful if there is no CIFS share providing access to the VMDK file.
Name Type Size Created Modified Owner Group Perm Empty
---- ---- ---- ------- -------- ----- ----- ---- -----
/vol/vol2/cifs/VHDX/win2k8r2.vhdx file 25 GB 8/16/2012 8/16/2012 0 0 1077
-------------- Example 4 --------------
C:\PS>ConvertTo-NaVhdx -SourceLun /vol/vol2/luns/lun0 -DestinationVhdx F:\VHDX\fromLUN.vhdx
Convert the Data ONTAP LUN into a VHDX.
Mode LastWriteTime Length Name
---- ------------- ------ ----
-a--- 8/16/2012 3:38 PM 10775166976 fromLUN.vhdx