Dism /Unmount-Image and Files Explorer problem

Hi

I am using Windows 8.1 Update with its Windows ADK.

I noticed that if I run the command

Dism /Unmount-Image /MountDir:"<Mount_Directory_Path>" /discard

while the Files Explorer is opened on any directory, this command displays an error and it can not unmount the volume image. Instead, if I close the Files Explorer, the above command unmounts the image correctly. How come?

Thanks

Bye

Balubeto