I used the above modded bios to successfully flash my Gigabyte GA-H77M-D3H (rev. 1.0) and installed the pcie nvme adapter on the 2nd PCIE x16 slot (manual said that this PCIE slot will operate at PCIE2) and the benchmarked speed is around 1,500MB/s which is the actual (real world) limit for PCIE2 (theoretical limit for PCIE2 is 2,000MB/s).
But I ran into a big problem. When I installed a Nvidia 1660s GPU on the first PCIE3 x16 slot, Win 10 will not post (not even to bios). When I unplug the GPU, it will then boot into Win 10 successfully. At first I thought it might be that I initially cloned my Win 10 install from a previous install on a HDD, but I did do a clean install Win 10 afterwards to make sure that wasn’t the problem. If I manage it boot it successfully with the GPU & its drivers installed (it will randomly be able to post but rarely), it will hold if I just restart (reboot) the system, but it will fail and not post once I shutdown (power-off) the system and try to power-up next time.
I fiddled with different settings in the bios to no avail. Tried installing only pure drivers only without the Geforce Experience portion. I even installed Win 11 to see if that was a solution. Finally, to cut this short, I finally was able to get a stable boot with the 1660s & Nvidia drivers installed and it was by using another modded Gigabyte GA-H77M-D3H bios found on this forum in a August 2018 post by Kvicala at [HowTo] Get full NVMe Support for all Systems with an AMI UEFI BIOS
In Kvicala’s post will be a link to the modded bios. Inside the zip file, there are actually 2 modded bios, one with the name of “by FULL” and “by small”, but with no explaination of the differences in the info.txt supplied within. I tried the “by FULL” one and was successful in using that to get a stable boot with my 1660s GPU with the Nvidia drivers installed. Hope this helps will save someone down the line a lot of time.