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I'm trying to create a dual-boot system on my new Asus computer, which came preinstalled with Windows 8. I want to install Ubuntu.

The documentation says that if my Windows installation is in EFI mode then I need to also install Ubuntu in EFI mode.

I checked my BIOS setup and, it is, in fact, set to boot in UEFI mode, so I know my Windows installation is in EFI mode. But the question is, why I must install Ubuntu in EFI mode in this case?

The reason that I'm asking is that, The Asus BIOS is American Megatrends'. It calls Legacy mode "CSM" ("Compatibility Support Module"). When I turned the "CSM" mode on, my Windows 8 still boots. Moreover, with EFI mode, I wasn't able to boot my Linux from USB pen, but now I can.

Basically, my machine is capable of booting Windows 8 in EFI mode, and booting Linux-Live from a USB pen in the non-EFI mode. Why do I still have to install my Ubuntu in EFI mode?

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Because you want to use UEFI and have a GPT partition... –  Ramhound Apr 20 at 20:36
    
So? Sorry, I still don't understand why. –  xpt Apr 20 at 21:46
    
Further study shows, from Seven ways to set up multi-booting with Windows 8 and Linux, Enable 'Legacy Boot' The third "simple" possibility is to enable 'Legacy Boot' in the BIOS configuration, and just ignore the whole UEFI issue. This is not an option that I personally prefer, in part because I am stubborn and in part because as Adam Williamson explained to me some time ago there are some functional advantages to UEFI boot. But it certainly is a viable option, and strictly in terms of getting Linux installed and booting it might actually be the absolute simplest solution. –  xpt Apr 20 at 22:02
    
I Just explained why. Legacy Boot does not support GPT partitions. So if CSM is working then your partition isn't GPT. –  Ramhound Apr 20 at 22:17
    
From Booting from GPT (rodsbooks.com/gdisk/booting.html), it says that both Grub Legacy & syslinux support booting Linux from a GPT disk. So you mean if I install Grub Legacy/syslinux to GPT's PBR, UEFI can't boot them? I.e., UEFI can't boot Linux from a GPT partition? –  xpt Apr 21 at 2:47

2 Answers 2

up vote 1 down vote accepted

fooot is correct, but I want to elaborate on that answer a bit.

Most EFIs include a CSM, which enables booting older BIOS-mode boot loaders. The trouble is that the integration of BIOS-mode and EFI-mode booting is poor. Ubuntu, like many Linux distributions, uses GRUB 2 by default, and GRUB 2 can't switch from an EFI-mode boot to a BIOS-mode boot or vice-versa. In a pure-BIOS or pure-EFI setup, though, GRUB can present boot options for Windows (or other "foreign" OSes). Thus, the mixed-mode setup represents a step backward from the point of view of in-Ubuntu tools managing the boot process.

On some computers, you can use the computer's own boot manager to switch boot modes. In the best of cases, this works reasonably well; however, on most systems, this process is awkward (it requires hitting a function key at just the right moment during the boot process). On some computers, it's impossible -- their boot manager doesn't permit switching boot modes, or imposes arbitrary restrictions based on the partition table type, which makes it impossible to switch boot modes on a single disk. From the point of view of Ubuntu's developers and documentation authors, this solution is unsatisfying because it's outside of Ubuntu's control and so variable that it's essentially impossible to document. Saying "don't do it" is much easier and more helpful than writing dozens of pages trying to adequately document a procedure that won't even work on many computers.

In most cases, there are no disadvantages to booting Linux in EFI mode -- at least, not once the OS is installed. (Many people find the process of installing in EFI mode to be more challenging than installing in BIOS mode. That's a matter of a combination of EFI bugs, poor OS-installer design, and lesser user knowledge of EFI-mode than BIOS-mode quirks.)

If you're dead-set on mixing boot modes, you can always try it. If you luck out, your firmware might make it easy to switch boot modes. If not, look into my rEFInd boot manager. If you edit its refind.conf configuration file, uncomment the scanfor line, and ensure that hdbios is among its options, it may permit painless switching between boot modes. This isn't guaranteed, though; some EFIs lack the necessary support, and rEFInd's BIOS-mode boot options are still primitive. Furthermore, rEFInd boots Linux kernels directly in EFI mode, so by the time you install rEFInd, you'll probably have a cleaner and simpler path to a pure-EFI boot mode.

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Some BIOS implementations may only allow either all UEFI boot or all Legacy boot, and it would not be possible to mix the two.

Other BIOS implementations are able to deal with both. It sounds like the CSM mode allows you to boot both UEFI and Legacy drives. My motherboard does this, and so it has a "UEFI Hard Drives" and then a regular "Hard Drives" group, and so on. The drive will be in whichever category corresponds to its mode.

When installing Ubuntu, the mode you get will be related to the mode you boot with. If the USB will only boot in CSM mode, then it will probably install in a non-UEFI, Legacy mode.

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