In Windows 2012 server / windows 8, Microsoft introduced the new virtual disk format known as VHDX. We cannot add these disks to 2008 Hyper-v which supports the VHD format.

Here is a simple power shell trick do the conversion from VHDX to VHD.

PowerShell Convert-VHD –Path "D:\Master_Copy\Windows_2008R2_Evalution.vhdx" –DestinationPath D:\Windows_2008R2_Evalution.vhd 1 Convert-VHD –Path "D:\Master_Copy\Windows_2008R2_Evalution.vhdx" –DestinationPath D:\Windows_2008R2_Evalution.vhd