A. When you create a differencing disk, there's a parent-child relationship between the Virtual Hard Disk (VHD) files. The differencing disk contains only the changes from its master disk. If you want to merge a differencing disk back into the parent, you have several options. First, you can use DISKPART, as shown here:

select vdisk file=d:\temp\child.vhd

DiskPart successfully selected the virtual disk file.

merge vdisk depth=1

100 percent completed

DiskPart successfully merged the differencing chain.

Another option is to use the Hyper-V Manager. Select the Edit Disk action, select the child VHD, then select Merge for the action.

Merging a VHD differencing disk