This & RAID drivers are about the only reason I still have a floppy drive.. try the bootable CDR & hopefully it will work fine, just select no when it prompts you to backup the old BIOS, however I suggest placing a copy of your original BIOS on the CDR too just in case.
Asus & several other MB makers now include a built-in utility within the BIOS that will allow flashing it without any media involved in the boot process & some like MSI allow flashing from within Windows too, although I'd suggest against it.
Flash-drives can be made bootable, but I've never had a reason to do it so I can't offer any advice on how to set it up, also not all MB's support it... consult your manual for info.