Generally you would put the USB and or Optical disc first and second. When the BIOS sees that they are not present, they are instantly skipped and it goes to the next one, which would be your SSD. If your SSD fails to read your OS, then before you restart, you would put your OS disk or a bootable USB in for an OS repair or re-installing the OS. At this point, the BIOS sees the new bootable devise and use it instead of the failing OS on the SSD. You wouldn't have your WD HDD in your boot order since it doesn't have an OS on it.