UEFI and/or GPT format are not a requirement.
I was using a Win10XPE_x64 made using as source 10 x64 v1903 and respective Dism Version: 10.0.18362.1
I finally made FFU capture work, after reading this post:
I was having this exact same issue. It appears there is a bug in ADK v1903. The WinPE ISO it produces allows for imaging an ffu to a disk, but will not capture the ffu from a disk. If, however, you use the v1809 of the ADK, this problem goes away. I tried multiple times rebuilding the ADK using v1903 but ended up with this same error. The ISO from v1809 works out of the gate and will image OS versions at least up to Win10v1909.
I decided to
make a new Win10XPE_x64 using as source 10 x64 v1809 and also (EDIT: Previous comment is Not required) went to http://jfx.square7.ch/GWT/oldversion/and downloaded GetWaikTools_v18.10.zip, this version downloads Dism version 10.0.17763.1 (for 10 v1809).
After this, tried again booting this time from 10XPE_x64
(made from v1809) (EDIT: Previous comment is Not required) + Dism version 10.0.17763.1 (for 10 v1809) and using same USB stick, this time FFU capture from DMS was successful on my CSM/MBR PC without any issue.
So UEFI and/or GPT format are not a requirement.
EDIT-1: I found the link to GetWaikTools_v18.10.zip is not working, I will attach it here, password is GetWaikTools
EDIT-2: I found Win10XPE builds made from v1809 have some troubles when dealing with a USB device, it sometimes fails creating a new file or folder or renaming a file/folder located on the USB, so I decided to go back using as source 10 v1903 and no more troubles with this issue. Of course I'm using Dism version 10.0.17763.1 (for 10 v1809) on Dism Mount Service.
Edited by alacran, 10 April 2020 - 07:03 PM.