To unmount the disk
Login to HyperV Manager.
Right click on NSM 3.1.1HF1 VM. Click Settings.
The VM should be in shutdown state.
Select the Hard Drive under SCSI Controller.
If the attached file is in .vhd format, then convert to vhdx format. Follow steps under Converting Additional Data Disk from VHD to VHDX Format
Record the .vhdx file name and location.
Click Remove.
Click Apply.
Click Ok.