Trisquel GNU/Linux installs in legacy/CSM/BIOS (but not EFI) mode but does NOT boot.

Hello Trisquel GNU/Linux community,

I'm in the process of liberating a new laptop (Asus X451) from Windows 8. (Just in case this may help someone later, the WiFi chip in this computer is by Atheros and works with Linux-libre kernel). The computer in question has (U)EFI but allows the so-called legacy/CSM/BIOS mode to be enabled and also Restricted Boot (or "Secure Boot" as Micro$oft calls it) to be disabled.
Release notes for Trisquel 6.0.1 mention that it works with (U)EFI. So after wiping all the Windows partitions and dd'ing the Trisquel 6.0.1 image (the Gnome Desktop option) to a USB flash disk, I tried booting up from the flash disk with CSM disabled, which means that we are in EFI mode and the efi kernel modules must be loaded if things work. But the system does not recognize the image on the flash disk and does NOT boot at all. With CSM enabled, Trisquel boots up and I can try/install it without any problems, except that the installed system does not boot up. In a normal install, I have usually used a three-partition setup: one for /, one for /home and one for swap. That's what I did for this sytem too. I tried rewriting the MBR (Master Boot Record) using grub (after mounting the necessary devices on the installed system chrooting) but that didn't solve the problem, neighter did "boot-repair" which is basically a GUI for doing what I did manually. I also tried a four-partition setup with the extra partition being the first one and flagged as bios_grub in gparted. This did NOT boot either.

Here are my questions:
- Why is it that I cannot boot into the Trisquel flash disk in EFI mode?

- How can I get the installed system to boot up? I don't care about EFI and I'm happy with BIOS/CSM mode for now. I'm out of ideas.

- A flash disk with an image of the spyware distro Ubuntu 14.04 boots up in EFI mode just fine. I read on their website that images for 12.04.02 and later work in EFI mode. Does Trisquel 6.0.1 correspond to 12.04.01 or 12.04.02?