Convert physical disk (GPT) to VHDx

How to do it:

Use Disk2vhd to create a VHDX (or VHD, but I recommend VHDX) from your physical disk - it doesn't really matter which drives you check/uncheck as long as your Windows drive is checked.

Create a Gen2 VM in Hyper-V and attach the new VHDX and your choice of Windows installation media (DVD or VHD).

Boot the VM (it shouldn't matter what's listed first under the Firmware tab - it'll likely boot from your installation media anyway since your VHDX doesn't work yet).

Hit Shift+F10 to bring up the Command Prompt (or click through troubleshooting buttons to the Command Prompt).

Type "diskpart" and hit Enter.

Type "list vol" and hit Enter - note the drive letter of your Windows volume (most likely the largest option), as well as which volume says Raw next to it.

Type "sel vol #" where "#" is the number of the volume that says Raw next to it (this is the reason your VHDX doesn't "just work" when Disk2vhd has completed).

Type "format fs=FAT32" and hit Enter.

Type "assign letter=s" and hit Enter.

Type "exit" and hit Enter.

Type "bcdboot c:\windows /s s: /f UEFI" and hit Enter (where "C" is the drive letter of your Windows volume from the "list vol" command, and "S" is the letter we just assigned to the System volume after formatting it to FAT32).

Type "exit" and hit Enter to close the Command Prompt window, then restart your system.

That's It.