The only supported Windows on USB is Microsoft's To Go versions and what they use for installations.
What you are trying is unsupported.
So this morning I was trying to run Windows 8.1 Pro on my 64GB flash drive (files copied successfully), but when I tried to create the boot files on the drive it gives me an error saying: Failure when attempting to copy boot files. The command I ran was bcdboot.exe H:\Windows /s H: /f ALL where H: is my 64GB flash drive. Is there a fix to it? If yes then how? PLEASE HELP ME ASAP as I am trying to fix the HP Pavilion Laptop ASAP. Thanks
P.S the command was run in CMD as admin on my Windows 7 desktop.