How can I copy a VHD image back to a physical disc?

You can use qemu-nbd in Linux to access a disk image as if it were a block device.

Here are some examples of operations that can be performed from a live Knoppix terminal.

  su 
  modprobe nbd
  qemu-nbd --read-only --connect=/dev/nbd0 --format=vpc <vhd_file_name>

If VHDX format:

  qemu-nbd --connect=/dev/nbd0 --format=VHDX <vhdx_file_name>
  ddrescue --verbose --force /dev/nbd0 /dev/sda  # write image to /dev/sda

Write one partition:

  qemu-nbd --partition=2 --read-only --connect=/dev/nbd2 --format=vpc <vhd_file_name> 
  ddrescue --verbose --force /dev/nbd2 /dev/sda2 # write partition 2 of image to /dev/sda2

Mount partition:

  qemu-nbd --partition=2 --read-only --connect=/dev/nbd2 --format=vpc <vhd_file_name>
  mount /dev/nbd2 /mnt 

Unmount and disconnect image file:

  unmount /mnt 
  qemu-nbd --disconnect /dev/nbd2

There are at least two similar questions that got accepted answers:

https://superuser.com/questions/40294/copying-a-vhd-to-a-physical-disk

Vdisk (vhd) to physical

But you can't do it from a running OS.

It would be a hard thing to script this. Following the Linux dd aproach one could say that everything is possible. Boot from PXE, locate the VHD, locate the physical disc, dd from V to P, when finished reboot.

Use HDDGuru Raw Copy Tool

1. Mount the VHD(X) using explorer on Windows 8/10
2. Plug in the target disk
3. Use the Raw Copy tool to copy the mounted disk to target physical disk

This can be used for any of the mountable formats on Windows, iso, vhd, vhdx etc.

It is possible to use Virtualbox to attach a physical device as a virtual device inside the VM

Windows method

VBoxManage internalcommands createrawvmdk -filename "C:\Users\vhudson\VirtualBox VMs\usb.vmdk" -rawdisk \\.\PhysicalDrive1

Linux method

VBoxManage internalcommands createrawvmdk -filename usb.vmdk -rawdisk /deb/sdb

Using an additional iso as live CD something like hirens boot or some linux rescue image, it is also possible to do image copying from one virtual device to a physical device that is virtually present inside the virtual machine.