@bigchief Sounds like a mismatch of bios/ueif to me. A UEFI system can sometimes have the feature to optionally legacy boot (bios). What mode is the firmware in? (bios/legacy or UEFI)? So figure that out - and then dig through the firmware settings to make sure no optional Legacy modes are enabled.
Also, could be something as simple as turning off the HDD’s RAID mode in firmware and enabling AHCI mode.