You probably know the scenario:
The client has started a small project and needed a new server for this.
In these days you useally go for a VM running on a server, and (since it uses few resources) you place the disk content of that VM into a file in the filesystem.
Since it's a small project with small load this is the ideal (cheapest) solution.
A few months later, you realize that this project is not as small as the customer tolds you or is requiring more and more storage place.
So you add more physical disks to the server and make a dedicated mirrored drive for that (once small) VM.
And now you have to transfer the content of the VHD file into the real disk, so you can directly access it with best performance from the VM.
Since MS did not provide a tool for this conversion, you need another tool for it.
There are serveral of them, the simplest GUI tool is Vhd2disk from Sysinternals.
Here is the link to the forum with the download links.
The conversion is very simple, just make sure to select the correct target disk, otherwise you will miss some other data!
Once restored you can change the disk in the Hype-V manager and (if required) expand the volume(s) to take advantage of the additional space.