Comment 11 for bug 1905491

If you look at the freshly written image with Gnome disks , 1st ubuntu partition , click the cog wheel and choose edit partition you'll notice that the build team have not enabled the Legacy BIOS bootable option?

If you enable it and click change the drive will now boot , there is still a really long 20 second pause with a black screen and blinking cursor but it eventually swaps over to the Ubuntu boot logo and starts.