Ahh. EPYC doesn't use DMI, so there’s an easy elimination, and they have enough PCIe lanes so that switches only come up with boards supporting oodles of drives. It’s still worth checking your mobo manual to make sure there isn’t a specific mention of port selection related to PCIe switches, or a silly bios option.

There might be some confusion that’s holding you back on diagnostics though. If a PCIe switch was in the path and causing the issue then there’s no meaningful difference between “parallel” and “bidirectional.” The nature of the switch is that all the traffic goes through it and it has a top speed and that’s split among the operations. Read or write to a single drive gets full bandwidth, but copying from one to another is read and write, so each part gets 50%. Even on the same drive, write/read and write/read also get 50% each. Include other drives on the same switch and divide it again. “And” = divide.

Your platform makes that specific issue less likely, but hardware can be a bit more quirky than you might be giving it credit.

Of course you’ve turned off windows defender real-time scanning, but otherwise I can’t think of an on-by-default reason for Windows Server to cause that issue, but it isn’t inherent in the OS. I’ve used multiple versions in dozen GB/s arrangements - 1.5GB/s was beatable 20 years ago. There’s something going on with your settings or hardware. Good news is it might be fixable, bad news is you’ll have to go trekking around to find the issue. :/