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I've read https://help.ubuntu.com/community/UEFI but I'm still puzzled with the flow of boot process in UEFI mode for Ubuntu.

I understand that the first step is UEFI firmware will load bootx64.efi provided by Ubuntu which is stored in ESP. What this file does? Launching GRUB2? Is it safe to just copy bootx64.efi from one disk to ESP in another disk?

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I find the answer in http://wiki.osdev.org/GRUB_2. Looks like bootx64.efi itself is GRUB 2. The configuration file grub.cfg read by bootx64.efi is also stored in ESP. By default, the grub.cfg in ESP is a configuration to read another grub.cfg in partition where Ubuntu is installed.

  • The version of grub that is bootx64.efi, is not a full grub as in an install. I do copy the version of grub in an install to a full Ubuntu install on a flash drive or external device. But have to copy twice as the full install version of grub is hard coded to find the rest of the grub UEFI part of grub in /EFI/ubuntu. So I copy to /EFI/Boot on external and rename shimx64.efi to bootx64.efi. And copy again to /EFI/ubuntu on external.askubuntu.com/questions/559007/… Grub also has commands to directly install to ESP. – oldfred Jul 5 '16 at 3:45

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