My gut here is saying you have a mismatched combo of how Coreboot is treating these, and how they are written. From what I’m reading, Coreboot should support Legacy, UEFI, or SeaBIOS, so go set that in the BIOS setup, then make absolutely sure your disks are being written as such (NOT mbr). Ventoy should be the tool to use here for testing different distros out, so good on finding that.