Yes I had same issue on Asrock Z77 Pro4 BIOS. I couldn’t insert NvmExpressDxe_4 (too big) into DXE volume (csmcore) with MMTool, just inserting the small NvmExpressDxe_Small.ffs worked with MMTool.
However using UEFI-Tool 0.26 more manual method, e.g didn’t complain at all about inserting that NvmExpressDxe_4 int o Z77 Pro4 firmware volume, done exactly as Fernando described, after the last GUID of the found DXE volume
(UEFI Tool: ->File ->Open image file, small bar - select (All files (*)), klick on firmware file and klick open, expand double-clcking on "Intel Image", then double-click on BIOS region:
text-search for "DXE" - clicking on ->File ->Search (STRG + F), select bar ->text ->text field ,type in "DXE" (while "Unicode" checkbox enabled),
then on the listed result-search-output at the UEFI_Tool-bottom-bar-window, double click on anyone selected result
then on GUID-group-tree result, scroll down to last XXXXXXXX-XXXX-XXXX-XXXXXXXXXXXX of the found DXE volume-hirarchy group, klick on that last of within that tree group, see here in one of the few last screenshots of adding nVME module on UEFI Tool method description from Fernando) then ->Action ->Insert after, navighate the NvmExpressDxe_4, open)
Keep in mind I’m not 100% sure if that gets an OK, functional modded BIOS, wheh MMTool rejects doing it, not tested in use yet, (see take caution counter measures I can do if firmware gets bricked, see below!! (Please see below)
Coparing modded Asrock Z77 Pro4 2.00 firmware with original firmware, I couldn’t notice any bad modding so far (firmware containing also no pad files/padding afaik), as Fernando recommended to compare original and modded BIOS file, that there’s no existing GUID/module/etc got missing from adding an .ffs module.
Firmware chip is DIL/DIP socketed (removable) and I have I hardware programmer to recover it in case s.th. goes wrong, orifginal Backup BIOS image already made with TL866 external programmer, as (Flash descriptor is locked, (Intel FPT Tool wouldn’t allow dump it "The CPU doesn’'t have access to …please modify the flash descriptor setting…"- which would need Realtek Audio chip Pin mod to unlock flash descriptor for EEPROM SPI access)
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@Fernando :
Thx for info. Afaik it looks to me the nVME GOP driver is within the 8192-firmware, because when opening N24JU8M.14 with UBU (ubu.bat), it lists nVME GOP GUID when opening that 8 MebiByte firmware.
EDIT by Fernando: Unneeded parts of the fully quoted post removed (to save space)