Seemed no matter what USB stick I used, or how I created the bootable image, my ASRock J5040-ITX would fail to recognize the USB drive. That until I discovered that if I plugged the USB stick in to the black USB 2.0 ports on the rear panel I/O it works!
I was also having the same issue with updating the BIOS from 1.60 to 1.70 and once I put the USB stick with the new BIOS in the USB 2.0 port under the network port, it worked as expected. Posting incase anyone else is struggling with this issue.