Re: Disabling "legacy usb" in BIOS: problems
Check your drive boot order in BIOS. I think you typically need to have Legacy or Auto enabled for USB legacy support in order to boot from a USB drive, but maybe your system is trying to boot the USB drive when there is no OS on it. So that is why you should check your boot drive order.
How different computers handle that differs. I have my laptop set to boot USB before its internal drive, so if a bootable USB is connected, it automatically boots USB. But I have not tried booting that with a USB drive with no OS, or a boot loader and no OS.
For my old and new desktops and company laptop, regardless of boot order set in BIOS (other than which internal drive to boot), it will not even try to boot an internal drive unless I press a certain key during BIOS splash screen. That is probably to avoid accidentally booting from something (possibly malicious) if you do not intend to. The only virus I ever caught in Windows (3.1 and 95) was spread by floppy disks many years ago.
i5 650 3.2 GHz, 8 GB, nvidia GTX 1060, 32" 1080p & assorted older computers