Win8.1SE for ASUS UX501VW?

For Copy to USB-Device plugin, Add Grub2-EFI (x64 UEFI only) is only an option for multi-boot, to be safer, do not use it at first
For copy to USB or Copy to USB-Device BCD, the default options are OK with BootIce + FAT32.

To boot from an USB stick, Fast Boot should be disabled a priori. It may depends on PC manufacturer!

ChrisR, thank you. It seems to work, except for the Boot Option Priorities.
I have to change Boot Option #1 "[Windows Boot Manage...]" every time I boot the notebook.
The configuration change isn't lasting, quite annoying.

Half OT: The ZenBook comes with 4 partitions (#1: 260MB EFI; #2: 16MB MSR; #3: 237GB GPT; #4: 499MB WRE).
Image for Windows 3.03 will correctly offer these 4 partitions if I choose Drive Backup.
Acronis True Image 2017 merely detects 3 (#2 is missing) of them, so does TI 2016.

Good that it works 

Here on my acer laptop, I do not need to change the boot priority each time.
I can also start to the boot menu with F12 to select the drive or network for the startup.

Maybe you can try to start to the boot menu (ESC for Asus, I believe) and see if the USB stick is available.