Cannot boot WinPE, can boot Macrium Reflect USB. ASRock motherboard

I created USB bootable repair media. Diskpart tells me that each is MBR formatted, not GPT.
These were created on a Win10 machine.
One USB contains WinPE alone. I can boot it on my Lenovo Thinkpad, but cannot boot on my desktop that has ASRock H170M Pro4 motherboard.
Another USB was created with Macrium Reflect and it contains some version of WinPE, but seems incomplete (in particular I do not have MBR2GPT.exe). This USB will boot on either machine.

The WinPE boot failure on the ASRock system goes like this:
- I am give a choice of either USB or UEFI:Partition 1 for the USB drive.
- If I choose USB, it hangs and does nothing.
- If I choose UEFI, then it just comes right back to the boot choice menu.

Can anyone give me a clue why a perfectly good USB drive with WinPE will not boot on one machine, but is OK on the other?

What BIOS settings have you tried?

Try these steps and see these links:

Boot from USB Drive on Windows 10 PC Windows 10 Installation Upgrade Tutorials:
Boot from USB Drive on Windows 10 PC | Windows 10 Tutorials

If you have fast boot or ultra fast boot enabled in your UEFI firmware settings, then you will need to temporarily disable fast boot or ultra fast boot to be able to boot from a USB.
How to Enable Fast Boot or Ultra Fast Boot in UEFI Firmware Settings for Windows:
Enable or Disable Fast Boot in UEFI Firmware Settings for Windows | Windows 10 Tutorials

1. Try another USB stick
2. Try all USB ports (2.0 and 3.0)
3. Maybe when using UEFI Bios you have to temporarily switch to Legacy in BIOS settings
4. Turn off temporarily fast boot, secure boot, CSM if enabled

Another USB was created with Macrium Reflect

Rename the sources\boot.wim to macrium.wim.

One USB contains WinPE alone.

Copy boot.wim from that usb to your macrium usb sources folder.

Now check if it boots.

I turned off CSM, then copied boot.wim from WinPE to Macrium USB as suggested. I cannot find a Legacy BIOS option.
It breaks my Macrium USB. The Macrium USB boots in UEFI mode, progresses to an all blue screen, then nothing more ever happens.
Restored Macrium USB to proper state and it boots into Macrium.
The WinPE environment that I included when I built the Macrium USB is dated sometime in 2016 and does not include MBR2GPT, but has tons of other Windows EXE tools. I think I need to recreate Macrium, but be more particular about choice of WinPE.
As I stated before, the WinPE USB attempts to boot in UEFI, but nothing happens - I just return to the boot selection menu.
When CSM is enabled I am allowed to choose these two USB drives in a mode that is simply named "USB", not UEFI", but neither will boot in this mode - they just create a black rectangle that partially blots out the ASRock boot menu screen.

1) See if you can boot using other USB drives:
a) Memtest86+ version 5.01:
Memtest86+ - Advanced Memory Diagnostic Tool
MemTest86+ - Test RAM | Windows 10 Tutorials
b) Ubuntu:
Create a bootable USB stick on Windows | Ubuntu tutorials
c) Boot rescue:
Windows 10 Recovery Tools - Bootable Rescue Disk - Windows 10 Forums