Your USB disk probably isn’t your “first” disk. Is it possible you are hitting some variation of this bug?
Does it work if you boot both the installer and the installed system in UEFI mode? You should be able to disable “legacy” mode in your BIOS settings. Also, I like to use the F12 key when booting to bring up the BIOS boot menu and make sure that I select the UEFI boot option.