Ok, so. My turn.
Some years ago I took an Asus P9D-M based 1u server home for playing around with. I planned to use it as a new pfSense rig for a while. Worked great. I decided I wanted to fill up the RAM, because cheap.
Enter nightmare.
12 RAM sticks, a BIOS reflashing toolkit, a new BIOS chip, and 3 eBay Asus P9D-M motherboards, and a whole-bunch-of-time-and-headaches later....I was still in the exact same position. What the !@#$ is going on?! Why JUST this 10g SFP card (or any x8 SFP card I tried). x16 GPU worked fine. Other x8 cards work fine. What is going on!
I bought a Mellanox x4 single port SFP card and cut my losses.
A year or two after that, someone on reddit messages me randomly, that they found this thread and it solved their issue.
I had long stopped using that server but assured them I would try this out anyway on principal as I still had the parts to try. I booted it up, installed Ubuntu, powered off, and put the card in. Sure enough, it turned on with 4 beeps as it used to. After 10 minutes of trying to cut and place the tape thin enough, I was ready to test.
Holy shit. It booted. Pulling over 9gbit across the LAN.
Y'all are geniuses. I can't thank you enough for getting this wildly crazy issue an ending for me. I have no use for the server, but this fix absolutely worked.