Following from https://trisquel.info/en/forum/i-am-interested-using-trisquel-7-or-8-my-korora-laptop-not-able-boot-my-usb-20-drive:

I made my USB drive UEFI-only, and it finally booted. Instructions are at https://askubuntu.com/questions/395879/how-to-create-uefi-only-bootable-usb-live-media. I used gparted to format my USB drive as one fat32 partition, then followed the instructions under "1.1. Example via terminal".

I will be adding this to the wiki.

Now, I am finally an official member of the Trisquel community.