Once I had one Acer ultrabook with mSATA and SATA drives. The "legacy" mode
boot list didn't contain the mSATA option, so if I wished to boot from the
mSATA SSD, I had to use UEFI mode (the ESP can be on either SSD or HDD).
I believe what you described was something similar: restrictions by the
firmware. (Maybe the manufacturer didn't want users to install operating
systems using legacy mode.) Unfortunately, starting Haswell, Intel
implemented one treacherous "feature": Boot Guard. It is impossible to fix
the broken-beyond-repair non-free firmware with this anti-feature present.
What I can suggest is to try the latest Debian (testing) Installer and
perform a minimalist installation. See whether you can boot it from UEFI
mode. Use the "Alpha 3" release of Debian Installer. I tested it on many UEFI
systems. (Note: Debian does have certain non-free firmware on its mirror
servers. Avoid adding the "contrib" or "non-free" repositories and you can
have a free/libre system.)
