I'm a bit confused as to how many slots the Asus ROG Strix X570-E board has. On the top side, it has 2 slots, but I remembered that there can sometimes be M.2 slots on the back. I went to check ASUS' website, and was met with this: Quote 1 x M.2 x4 Socket 3, with M Key, Type 2242/2260/2280/22110(... More on linustechtips.com

I am after a similar answer - can I run a PCIe x4 NIC in the third slot without limiting the GPU in slot 1 to x8. The X570 architecture seems to imply this will work. The Zen2 Ryzen supports x16 lanes for GPU, plus x4 lanes for M.2, plus x4 lanes to X570 PCH. (Also has dedicated SATA and USB controller on the CPU). The X570 PCH has x4 lanes to PCI x4 slot, shared with PCIe x1 slot, and x4 lanes to another M.2, and more to the NIC, SATA, more USB. So - you should be able to run what you want (theoretically), bearing in mind that the PCH M.2 and the PCIe x4 M.2 will be sharing x4 lanes to the CPU. However. The x570-E manual shows that the three slots can operate at x16, x8/x8, or x8/x8/x4. Notice that the third option gets x4 additional lanes out of ‘nowhere’, because these are off the PCH not the CPU. The first 16 lanes are off the CPU and can be used in x16 or x8/x8 configuration. I asked ASUS support, and they have said that using the x4 slot will put slots 1&2 into x8/x8 configuration, which is really annoying because there is no reason that I can see that this needs to be forced. I have asked in a number of places whether someone with this board can actually test if this is the case, but have not had a response yet. More on reddit.com

2 x M.2 Socket 3 with M Key Type 2242/2260/2280/22110 storage devices support (PCIe 4.0 and SATA modes) Yes it would support SATA m.2 although ideally you want the boot and OS drive to be nvme SSD - your 970 Evo More on reddit.com

Top slot is basically wired in directly to the CPU making it super fast and best used for the drive with OS installed in my opinion. The bottom drive passes through the x570 chip on the mb before the CPU. Gen 4 vs 3 isnt really a big deal. I run my gaming ssd on the bottom slot and it's gen3, it's lightning fast with high read write speeds though. Your good no matter what you decide to do. Leave the settings for your m.2 in bios to auto and dont worry about it. More on reddit.com