i know that's im too late but i suffered with this problem and i finally get it so i hope my answer helps someone
to get into x10rsa onbord raid you need to configure sata as RAID from SATA Tab in INPUT/OUTPUT menu then enable the HDDs those you want to set as raid then go to the next tab in the same menu its name is PCIe/PCI/PnP then scrool down to the bottom you will get option called RSTe configuration click on it and you will be able to create RAID VM after all this go back to SATA tab in INPUT/OUTPUT menu and disable all HDDs you enabled before and notice that you will need to load driver for RAID while installing windows you can downlad it from the official SuperMicro website support and it is named Intel PCH good luck
to get into x10rsa onbord raid you need to configure sata as RAID from SATA Tab in INPUT/OUTPUT menu then enable the HDDs those you want to set as raid then go to the next tab in the same menu its name is PCIe/PCI/PnP then scrool down to the bottom you will get option called RSTe configuration click on it and you will be able to create RAID VM after all this go back to SATA tab in INPUT/OUTPUT menu and disable all HDDs you enabled before and notice that you will need to load driver for RAID while installing windows you can downlad it from the official SuperMicro website support and it is named Intel PCH good luck