I think my fdisk output below indicates a MBR partition table with the "dos" Disklabel type. Right? Other USB drives I have show gpt Disklabel types. This USB boots Manjaro in UEFI mode, but not with Secure Boot.
Interesting ! After some reading it seems that MBR partition tables handling is required by the UEFI specs, which means you can theorically boot from MBR devices in UEFI mode. But some lazy manufacturer go directly to compatiblity mode when facing a MBR disk.
I was wrong, thanks for pointing it out !
Gparted shows sdf1 as iso9660 filesystem, with partition table = none. Gparted does not see the EFI partition at all.
The rEFInd drivers page states that there are some UEFI drivers for ISO9660 out there but its quite uncommon. Are you sure the key is booted in UEFI mode ? Having an installer booting in legacy mode won't prevent it from installing a system for UEFI style boot (only asking for the sake of improving my knowledge).
Gparted does not see the EFI partition at all
EFI partitions are FAT partitions at this time. They forked the specs from FAT to have more freedom on what they can change about it. Basically an UEFI system can boot from any partition he can read. EFI partitions are guaranted to be handled by UEFI, the rest is optionnal.
An UEFI system will check partitions it can read an look for an OS loader at a fixed path (/EFI/BOOT/BOOT<MACHINE_TYPE_SHORT_NAME>.EFI ).
But you can, on properly designed systems, set another path manually.
BTW don't hesitate to point out if I'm wrong. I didn't take the time to read the gigantic full UEFI specs at that time, I'm trying to solve the puzzle with small parts grabbed arround.
Gentlemen, problem solved.
\o/