Do both Windows and Linux have to be in legacy boot mode during dual boot?

Question

I have just bought a Dell Inspiron laptop which supports both UEFI and legacy boot mode. I already have Windows 10 installed in legacy boot mode. Do I need to install Ubuntu in legacy boot mode or UEFI mode for dual boot? Which mode will be better?

Answer 1

On most computers, two OSes don't have to be in the same boot mode, but it's helpful -- very helpful -- for both of them to be in the same boot mode.

Details vary greatly from one EFI to another, but in all but some very ancient EFIs, BIOS-mode OSes are installed using a Compatibility Support Module (CSM), which is a sort of BIOS emulator for EFI firmware. The CSM can be switched on and off, either by you setting options in the firmware or by the firmware itself, when it detects whatever key things trigger it to use the CSM or not use it. Unfortunately, there's very little standardization in the way different EFIs (and therefore computers) determine which boot mode to use when both are active. This makes the CSM a temperamental and tricky thing to use. That said, there are typically two ways that OSes using different boot modes can be managed:

• Using the built-in boot manager -- All EFIs provide some sort of built-in boot manager, but details on how they're used vary from one machine to another. Typically, you access the boot manager by hitting a function key (usually F8 or above), Del, or Enter soon after the computer powers on. This action produces a menu of boot options. Some of these will boot the computer in EFI mode and others in BIOS mode. EFI entries are usually either named by the OS that installs them (e.g., ubuntu or Windows boot manager) or include the string "UEFI." BIOS entries are likely to describe BIOS boot media, like be the name of a hard disk. Thus, you can switch between a BIOS-mode Windows and an EFI-mode Ubuntu by selecting the hard disk by name vs. an entry called ubuntu in the boot manager.
• Using rEFInd -- My rEFInd boot manager is a standalone boot manager that can do something similar to what the EFI's boot manager can do, but it works in more-or-less the same way on any EFI-based computer. (Macs are an exception; the CSM in Macs is unique, and so rEFInd treats Macs differently.) By default, rEFInd's CSM support is inactive, so you won't see BIOS-mode boot options. If you uncomment the scanfor line in refind.conf and ensure that hdbios is among the options, you'll see a gray diamond-shaped icon for your hard disk, which will launch any BIOS-mode boot loader it contains. This is in addition to whatever EFI-mode boot loaders the disk contains. Thus, you can switch between BIOS-mode and EFI-mode boot loaders.

Neither of these tools is guaranteed to work; some EFIs lack CSMs and so can't boot in BIOS mode at all. (Clearly that's not the case for you.) Also, some very early EFIs for x86-64 were actually implemented on top of BIOS, so they boot in BIOS mode by using the underlying BIOS. These EFIs don't have a CSM, so the boot manager works differently and rEFInd can't boot in BIOS mode. Chances are you don't have such a system, but it's possible you do.

Note that the CSM must be active for it to be useful, but activating it does not (usually) guarantee that it will be used. This is what makes switching boot modes from one boot to the next, without mucking with firmware settings, possible. That said, if your computer has nothing but BIOS-mode boot loaders installed, booting in EFI mode becomes impossible (unless or until you plug in an EFI-bootable medium or install an EFI boot loader). Once again, I'd like to emphasize that there are subtle (and not-so-subtle) differences between computers, which makes providing generic advice very difficult.

As a practical matter, this information is largely academic for you. Given an existing Windows installation in BIOS mode, you should probably install Ubuntu in BIOS mode, too. The trick is to know the mode in which the installer boots. That information isn't exactly identified in flashing red letters, so it's easy to boot in the wrong mode and not realize it. You may want to drop to a shell and look for a directory called /sys/firmware/efi. If it's present, you've booted in EFI mode; if it's absent, you've booted in BIOS mode. If you accidentally boot in the wrong mode, stop and try again.

Alternatively, you could convert the Windows installation to boot in EFI mode using Microsoft's new MBR2GPT tool and then install Ubuntu in EFI mode. This is likely to be a cleaner solution in the long term, but in the short term, it adds some risk (I'm not sure how much yet; the tool is new enough that I have no idea of how risky it is) and is likely to be more time-consuming than doing a BIOS-mode install of Ubuntu.

Answer 2

When dual booting, both OS's have to be booted in the same mode.
It does not matter which. If the two OS's are in different modes, they wil not see each other and you will have to switch boot mode in system setting each time you switch OS.

Since windows is already installed in Legacy mode, ubuntu needs to be installed in Legacy mode.

If you really want UEFI (there are advantages to it), then reinstall windows in UEFI. And Install ubuntu in UEFI mode.

NOTE: yes it is less hassle in most cases to install windows then ubuntu.

Answer 3

Well it is actually possible. I have a windows 8.1 and ubuntu 17.04 on my HDD hard which both have UFEI boot systems. On the other hand I have windows 10 on my internal SSD hard drive which runs on legacy boots on the same machine. If you install Ubuntu after the other OSs, grub will be able to recognize all your OSs and boot systems. However, if not you shall download boot-repair and apply its automatic fix. Therefore, like me, you get a grub with all the options you desire. Remind that grub-customizer can affect the boot-repair adversely (in case you use it). Moreover, you need a large EFI partition (about 200MB) as boot-repairs might occupy it.