Assigning drive letters to Volumes appears not to work

I tried to assign a drive letter to a new volume with diskpart. This doesn't seem to be implemented. I also used PartitionWizard to assign a drive letter but this also failed silently (no error message but also no drive letter). At least with PartitionWizard assigning drive letters should work. Here is a screenshot of  registry entry MountedDevices (on reactos):

I tried ls /cygdrive/d from my Cygwin shell, dir d: from a cmd shell and My Computer in ReactOS explorer all three commands failed with No such file or directory.