installer in UEFI mode, but existing operating system installed using "BIOS compatibility mode"

[cronyc]

So I wanted to install Linux Mint but I got this warning:

"This machine's firmware has started the installer in UEFI mode but it looks like there may be existing operating systems already installed using "BIOS compatibility mode". If you continue to install Debian in UEFI mode, it might be difficult to reboot the machine into any BIOS-mode operating systems later.
If you wish to install in UEFI mode and don't care about keeping the ability to boot one of the existing systems, you have the option to force that here. If you wish to keep the option to boot an existing operating system, you should choose NOT to force UEFI installation here."

Unfortunately I don't know what to do now. Can anyone help me?

[gm10]

Well:

If you wish to keep the option to boot an existing operating system, you should choose NOT to force UEFI installation here.

What's not clear about this? Don't install in UEFI mode if you want to dual boot the other OS.

[michael louwe]

@ cronyc, .......

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Most modern 2012+ UEFI computers give you the option to install an OS in UEFI or Legacy BIOS mode.
....... Eg to install in Legacy BIOS mode, go into BIOS Setup(usually by pressing one of the F buttons during startup), enable CSM(Compatibility Support Module) or Legacy BIOS and then boot the OS Install media/USB/DVD in Legacy BIOS mode, ie not in UEFI mode.

To dual-boot on an internal hard-drive, both OS should be in the same mode, either UEFI or Legacy BIOS, so that the active bootloader will give you the option to dual-boot either OS from a boot menu during startup.
....... You can force a dual-boot in mixed mode, ie one OS installed in UEFI mode and the other in Legacy BIOS mode, but this will result in the need to use the Boot menu of BIOS Setup to boot either OS.
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P S - The Rufus program for creating an OS Install USB requires you to specify either UEFI/GPT or BIOS/MBR or UEFI+BIOS = the BIOS Setup of the target computer should be compatible with the Rufus setting.

[kukamuumuka]

Simple check: If command sudo parted -l | grep msdos returns msdos , do not install in UEFI mode.

Edit: Of course if you install from USB-stick, partition table in the USB-stick can be msdos, so ignore that message. Better to check all disks if msdos is available.

Code: Select all

sudo parted -l

[BG405]

I use this one:

Code: Select all

[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS

Note that I'm using GPT for all newly-prepared hard disks, even though, currently, all but one of my machines are non-UEFI. The above command works either way.