I have a Ryzen 9 7900X with the Strix X670E-E board
I wanted to use 2 M.2s on an ROG Hyper card and a GPU, along with 4 more M.2s on the board, but I couldn't find anything in the manual about many things. And I'm a quite regretful and a little frustrated because of how simple this should have been, I don't recommend this board for any workstation users who need many nvme drives.
This board has 3 x16 slots and 4 M.2 slots.
X16_1(x16/x8/(x4/x4x/x4/x4) and X16_2(x8/x4) are connected to the CPU, while X16_3 is x4 connected to the chipset.
M.2_1, M.2_2, M.2_3, M.2_4 are all x4, while M.2_4 is the only M.2 connected to the chipset. It took a while to find but buried in small text in the manual there's a note that says if M.2_3X16_2 will drop down to X4 too.
To use the Hyper card. I have to use the first slot and set PCIe RAID mode for X16_1.
The row snippet from bifurcation support chart bothered to say it supported both X4/X4/X4/X4 and X4/X4, so I assumed 8 lanes would be free.
However, this does not seem to be the case on BIOS 1416. Enabling PCIe RAID Mode to enable bifurcation disabled both X16_2 and M.2_3. I don't see any drop-down menus or toggles to set it to x4/x4 raid mode either.
Thanks to that I've figured out X16_1, X16_2, and M.2_3 all share the same 16 PCIEe lanes. If I want to use 6 nvmes, instead of the simple x4/x4 Hyper card, I'll have to buy two more low profile pcie adapters to avoid impeding GPU airflow on top of using the chipset for one nvme array. It's that or buy threadripper for the privilege of decent bifurcation. $500 for braindead manuals and support pages on top of the exploding cpu scare and warranty fiasco, ASUS really knows how to kill their reputation for me.