value is 4 instead of 1 to prevent the virtual disk from expanding when mounted.
if the disk doesnt expand on boot, how can data be written to it?
This is from the part of the article related to WinToUSB
On a previous paragraph it says:
However, our Windows To Go drive loaded to a blue screen error that read "VHD Boot Host Volume Not Enough Space" and required a registry modification to the system inside the virtual hard drive, making as much or more work out of the WTG creation process than our manual method above.
It seems to me for some reason the expandable VHD size is bigger than the space available on the drive where it was created, then the need to apply that setting on the registry.
I don't know how the VHD is created since I haven't tested the mentioned tool yet, but it comes to my mind Hyper-V standard (preselected) size when creating a new VHD or VHDX (fixed or expandable) is 128 MB and this may be the cause if the host drive is not big enought to hold this expandable VHD.
Then it seems to me to use this tool we need a big size USB device with plenty free space, as a USB SSD or HDD not a limited USB stick.