I also want to boot from an USB. I have been reading your thread and like the clarity and precision in your posts. So I wanted to ask you if you solved the problem and how. In my case, a bit different, I'd like to boot a linux systm from an USB in a PC with grub1 (or legacy grub) installed and no bios support for booting the usb (at least there is no option in the boot orders) and I'd like to boot an Apple, with dual boot and grub2 installed in a partition. The apple, before passing through grub, at least recognizes that there is a linux in the USB at boot but fails to boot saying there is no support to booting external devices.
These are the threads, If you have something to say I would be grateful:
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