@ MartiniGM:
Welcome at Win-RAID Forum!
I have never had a problem to flash a modded v2104 BIOS into the BIOS chip of my ASUS P8Z77-V, when I used the USB Flashback method.
The USB Flashback method should work nevertheless.
I don’t think, that this is necessary.
Questions:
1. How did you try to flash your modded BIOS?
2. What has been updated/modified within the BIOS?
Regards
Fernando