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Per this answer on how to create an EFI System Partition (ESP):

(Quite likely though that the install disc itself must be booted in EFI mode, too, otherwise it won't bother setting up UEFI boot... in fact, certain parts of the setup can't even be done from BIOS mode.)

How do you boot the install disc in EFI mode? Assuming a USB flash thumb drive for the install media.

In the context of dual-boot with Windows 10.

Note that, as I read the question, the ESP was borked and the question was how to repair or recover it. It's not necessary to read the related question, it's as much for my reference.

  • So, you expect people to go to the superuser.com site, figure out the context of your question, then return here to post an answer? How creative. – Satō Katsura Dec 17 '16 at 9:20
  • not at all, I don't think that further context is necessary to answer. – Thufir Dec 17 '16 at 9:35
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Quoting from the debian installation manual:

"...The latter becomes important when booting debian installer on a UEFI system with CSM because debian installer checks whether it was started on a BIOS or on a native UEFI system and installs the corresponding bootloader. Normally this simply works but there can be a problem in multi-boot environments. On some UEFI systems with CSM the default boot mode for removable devices can be different from what is actually used when booting from hard disk, so when booting the installer from a USB stick in a different mode from what is used when booting another already installed operating system from the hard disk, the wrong bootloader might be installed and the system might be unbootable after finishing the installation. When choosing the boot device from a firmware bootmenu, some systems offer two seperate choices for each device, so that the user can select whether booting shall happen in CSM or in native UEFI mode...."

So, disable the legacy mode (also called compatibility mode or CSM) from BIOS and try to boot into the already installed OS, if there is any. If the boot is unsuccessful, the OS on your hard drive was installed in legacy mode and you can't boot or install another system in EFI mode. Else, go ahead and install the OS.

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When a distrie supports uefi, in the efi-menu is your installation-medium USB-Stick/DVD shown twice. There you can choose to boot in uefi-mode.

After boot you can check it with mount | grep efivars The command should have an output like this. /sys/firmware/efi/efivars Before you install linux disable fast boot in uefi and windows 10. Link here

  • That's fine. Would you please elaborate and clarify the first sentence? Where is it shown twice? Why? BIOS and UEFI? How do I know which is which? – Thufir Dec 17 '16 at 12:32
  • I'm sorry the allocation depending on your uefi-menu. <help.ubuntu.com/community/UEFI Seek advice in your handbook . – user192526 Dec 17 '16 at 13:54

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