[syslinux] Is efiboot.img required?

[Ady]

> The most annoying thing (to me) is that it doesn't boot at all on
> Legacy OS mode (MBR), while it should... :-/
 
No, it shouldn't. If you are initially booting in (U)EFI mode (into 
rEFInd, or some EFI shell or any other EFI boot loader or EFI boot 
manager), then generally speaking you are expected to boot in EFI mode. 
But some (U)EFI firmware _might_ be able to "automagically" boot some 
external media (e.g. USB flash drive) in CSM mode without having to set 
it in the firmware.

To be clear, "recognizing" the USB device in the boot list is not the 
same as being able to boot from it. In general terms, you need to set 
your firmware in CSM mode so to be able to boot in such mode.

But that's not the only reason your procedure is failing. The method 
you described in your prior email (copying some files and renaming 
them) is not enough so to make your USB device bootable by SYSLINUX. I 
don't even know where you read such instructions (and please don't 
interpret this as an invitation to point to some guide that has 
incorrect instructions; let's not propagate inadequate information).

In your prior email you have not mentioned executing the 'syslinux' 
(or, alternatively, the 'extlinux') installer command, which is 
required to install SYSLINUX (on your MBR-partitioned with one FAT 
volume USB device) for BIOS systems.

And this is only about BIOS booting, before even considering the 
UEFI-related issues.

I would suggest reading:
 http://www.syslinux.org/wiki/index.php/Install 

Regards,
Ady.