I have recently purchased a new laptop and been researching the best ways to dual boot in Windows 8.1. I chose to dual boot from a VHDX file so I could either boot directly into my sandbox OS or run it from within Hyper-V should I have booted into my main OS.
The first step to do this was to install my main OS, then once installed open up Hyper-V manager and build myself a VM in the normal way.
The next step is to browse to the location on the hard disk where you have created the VMs hard disk. Once located, right click on the VHDX and select mount to mount the VHDX as a local drive.
Once you have mounted the drive, take note of the drive letter assigned to it.
The next step is to open an admin command window and using the BCDBOOT command run:
bcdboot <drive>:\Windows
Where <drive> is replaced with the letter of the mounted VHDX, in my case:
bcdboot F:\Windows
Once that has completed successfully, eject the mounted drive by right clicking on in and selecting eject. Now you can reboot and should be offered the option to boot to the VHDX OS as well as the main OS.
It is possible to run MSCONFIG and change the default boot and boot time. Its also possible to use BCDEDIT to change the description displayed on the boot window, as well as many other setting.
NOTE: Always make a backup with BCDEDIT before making changes.
To list boot options:
bcdedit
To change the description of a boot option:
bcdedit /set {boot loader identifier} description “Windows 8.1”
To list more information about the command:
bcdedit /?