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A couple of days ago I decided to (re)install Windows 10 on my box, which was running only Debian, but I couldn't get the UEFI to boot the installer from the various USB, or even a DVD, I prepared. Nevertheless, Legacy mode was able to do so and hence I decided to install Debian, Windows 10 and FreeBSD in BIOS mode.

Is there a way to get back to UEFI keeping GRUB2 and the three OSs bootable, without reinstalling them?

Another question I'd like to be answered is: booting in Legacy mode has any impact on the performance of any of the OSs?

Thank you very much for your answers.

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Yes; see this blog post for instructions for Windows 7. Windows 10 should be similar. The general procedure is to convert the partition table from MBR to GPT and then to install the Windows boot loader to the EFI System Partition (ESP). This is scary and a little tedious. There's also some danger to your data, but it's not that great if you're careful. Still, I strongly recommend doing a complete backup of your personal data before proceeding.

For Debian, you'll need to install a fresh EFI-mode boot loader. Boot Repair may be helpful; or see my page on EFI boot loaders for Linux for information on additional options. I'd do this after converting Windows to boot in EFI mode.

Since this is a fresh install, you may be better off just re-installing everything, unless perhaps you've devoted a lot of time to customizing your installation(s). If you're having problems booting the Windows installer in EFI mode, you should tackle that problem, since it's pretty fundamental. See my page on the Compatibility Support Module (CSM) for some tips that may be helpful. (That page is written mainly for Linux users, but the basic principles apply to Windows.)

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