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I have a computer (HP Pavilion g6-2342dx) which came with Windows 8, and thus the new UEFI boot system. I would like to install a few OS's which are not EFI compatible, and Arch GNU/Linux with BIOS mode. However, I would not like to completely wipe all my files to set to legacy. My preferred options would be to: 1. Multiboot legacy OS alongside my EFI os 2. Convert Windows 8 and Ubuntu to legacy without reformatting or switching to MSDOS part table.

If anyone could suggest a solution or a method, your help would be appreciated! Thanks!

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You can't. You will have to decide convert your HDD to an MBR partition and use the legacy boot or use UEFI and use GPT partitions. You cannot do both. Most operating systems including Ubuntu support UEFI so the decision is easy, use the one, that will be supported in the future by most operating systems and use a virtual box for everything else. –  Ramhound May 27 at 0:55
    
I'm sure that: 1) CSM supports GPTs, as my friend has a computer running CSM using GPT. and 2) things such as rEFInd have support for booting EFI and CSM side by side. However, I need clarification. –  The Great Widi May 27 at 22:08
    
I guarantee you your friend is not booting a GPT partition while in legacy mode. –  Ramhound May 27 at 22:32
    
    
You linked to a Wikipedia article that explains how GPT partitions work. –  Ramhound May 28 at 0:06

2 Answers 2

There are many solutions to this when you Google "Disable Secure Boot UEFI". I usually direct people to the HowToGeek article which gives a lot of details about the Why behind Secure Boot mechanics as well as How.

How to Boot and Install Linux on a UEFI PC With Secure Boot

One option that it lists that you may not have thought about is booting from a removable USB hard disk or thumb drive.

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I don't want to disable Secure Boot. I already dual booted Windows and Ubuntu under EFI/SB. I just need to know how to multiboot EFI os and CSM os side by side. In addition, the OS is not GNU/Linux. –  The Great Widi May 27 at 22:10
    
On some computers, using BIOS mode requires disabling Secure Boot. This isn't universal, though. –  Rod Smith May 31 at 18:51

AFAIK, the easiest way to do this is to use my rEFInd boot manager, which enables switching between EFI-mode and BIOS-mode booting. You'll need to uncomment the scanfor option in refind.conf and ensure that hdbios is among the options. Also, be sure to use the latest version; pre-0.8.0 versions had weak BIOS-mode support on PCs, and I made some improvements in 0.8.1 (the latest as I write) that are worth having.

That said, many BIOS-only OSes won't boot from a GPT disk, which means you'll need at least two physical hard disks to do the job. You should be able to do it with one disk if your BIOS-mode OSes all support GPT, though. (Linux and FreeBSD both do, for instance.)

Also, I don't see much point to booting Linux in BIOS mode if you're dual-booting with an EFI-mode OS unless you have specific problems with EFI-mode booting. (Proprietary video drivers sometimes have problems in EFI mode, for instance.) You'll boot more quickly in EFI mode, and you'll have more choices in boot loaders and boot managers. If you use rEFInd to select the boot mode, booting in EFI mode means you can eliminate GRUB and all its flakiness. (I'm not a fan of GRUB.)

Converting Windows from EFI-mode to BIOS-mode booting is another possibility, but I know of no site that documents the process. It would be the opposite of the procedure described on this page, though -- that page describes how to convert from MBR/BIOS-mode booting to GPT/EFI-mode booting.

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