Unmounting a VHD volume means removing the letter assignment for this volume.
To remove an existing drive letter from a volume, follow the instructions below.
2. Select an attached VHD in the "Attached VHDs" pane.
3. In the right pane, right-click a volume contained in a mounted VHD. Select the "Dismount" option from the appeared menu.
4. Confirm your request to unmount the Volume by clicking "Yes" in the appeared popup.
5. The drive letter is removed from the Volume that you specified.