I had the same problem with my GA-970A-D3 motherboard. It was not possible to boot from USB (stick or USB card).
I found out that the reason for this behaviour is the formatting of the USB medium.
I tried many formatting tools but only the "HP USB Disk Sorage Format Tool" succeeded.
So my way to get a bootable Ubuntu USB medium for my GA-970A-D3 motherboard is
1. Format the USB medium with "HP USB Disk Sorage Format Tool"
2. Use "Startup Disk Creator" or "UNetbootin" to get it bootable with an Ubuntu distribution.