OK so 90 views & 0 response later, apparently no ones knows.
So I'll answer my question myself (because, yes, I found the solution). For AMD systems like mine, when you set up your SATA drives as RAID just initializing the drive is not enough. You gotta create an array no matter what. When...
So here's my situation.
I'm configuring a new computer. I started with only my 2 NVMe drives plugged in, set RAID controller in BIOS instead of AHCI, installed windows 10 on one and then formatted the second one in Windows' Disk Management.
I then went and plugged my 3 SATA drives, two 2Tb HDs...