Does your BIOS allow you to completely remove a device from the boot list? If you can do this, do so. Set your Hard Disk and Optical Drive as the only boot-able devices. Everything else as disabled. As per USB Legacy Mode, it's only for devices not originally made to run in USB or are otherwise not USB aware being used in operating systems older than Windows XP. If this has a new reader I'm positive that it will not have any effect.