If SSD has nothing on it, it cannot be a bootable media.
Does your USB DVD drive have external power connection and is it connected ?
In some instances, USB power alone is not sufficient to give the disk a spin in the USB DVD drive. What is more, some boards are just not able to supply enough wattage to USB devices.
I suggest you to connect a 5-volt power adapter to the USB DVD drive and try again.
Hope this helps.