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.vhdDISKPART>
DiskPart successfully selected the virtual disk file.
DISKPART>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.