I try to install Ubuntu 16.04 next to Windows 10. Windows is installed on legacy bios and then upgraded to Windows 10. When I try to install Ubuntu from USB stick, it shows a message titled "Force UEFI installation?". It says that the machine's firmware started the installer in UEFI mode but there is an OS installed using BIOS compatibility mode. If I force installation I won't be able to boot Windows.
How can I start the installer in legacy mode?
BIOS of the machine supports legacy and UEFI modes. Legacy support is enabled. But USB disk is always recognized as EFI. USB disk's name is shown as "EFI USB Disk" in BIOS. I cannot change that. I burned the USB with both startup disk creator on ubuntu 14.04 and pendrive software on Windows. Same result.
Thanks.
Best Answer
You've got a BIOS/CSM/legacy-mode installation of Windows, but you booted the Ubuntu installer in EFI/UEFI mode. The two modes can be made to coexist, but doing so is tricky. It's generally better to boot the Ubuntu installation medium in BIOS mode. Broadly speaking, this can be done in either of two ways:
EFI/BOOT/bootx64.efi
for an x86-64 system.) If you're booting from an optical disc, adjusting the boot loaders on it is much more complex.Note that these two approaches are intertwined; you can't boot to BIOS mode if the boot medium lacks a BIOS-mode boot loader, no matter how hard you try. Thus, if you have trouble controlling the boot mode, you may need to re-examine how you prepared the Ubuntu installation disk, since that method might have omitted the BIOS boot loader. Also, some computers offer such poor boot managers that you may not be able to control their boot modes except by adjusting the boot medium.
If you need more information on this subject, see this page of mine. Note that it's written mainly for people who have EFI-mode Windows installations and who might be accidentally booting the Linux installer in BIOS mode, which is the opposite of your problem; but the basic information is still relevant.
Another way to solve your problem is to convert your BIOS-mode Windows installation into an EFI-mode installation, as described on this blog post. You could then install Ubuntu in EFI mode. In the long run, this may be a cleaner solution than booting both OSes in BIOS mode, for the reasons described on my CSM page; however, this approach will be much harder and riskier in the short term. Thus, it's probably better to stick with a BIOS-mode Ubuntu installation for now unless you have some other compelling reason to make the switch.