Can you check whether you have RAID controller on a integrated slot or on a PCIe Slot? If it is on a PCIe Slot then you can try disabling that slot in BIOS. 
Hello, I need to disable Integrated RAID controller on iDRAC 9 and cannot do that. On iDRAC 7 there is an option: Integrated Devices -> Integrated Raid Controller -> Disabled but it doesn't appear on iDRAC 9. What can be done ? Thanks in advance, Leon
Cannot disable Integrated RAID controller on iDRAC 9