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I'm an Ubuntu user and lover. Yesterday, I decided to install Xubuntu on my sister's laptop (which had Windows 8.1 preinstalled, in UEFI mode). It's a Lenovo G400S, 4 GB of RAM, Intel i5 CPU (I don't know the exact specs, sorry). But there was an issue: I could only boot from USB in legacy mode.

Note that I didn't do it the "regular" way: instead of burning the Xubuntu ISO image to the USB drive I put a bunch of ISO images from different Linux distros inside it and installed GRUB to its MBR. To boot Xubuntu Live media (64-bit), I created a loopback device with its ISO image mounted, loaded the kernel (at /casper/vmlinuz.efi) and the initrd (at /casper/initrd.lz) and it went fine. I created a 400 GB ext4 partition and installed Xubuntu there. Everything was correct and the installation successfully finished, but it was in Legacy mode.

Now if I set the BIOS configuration to give priority to UEFI it boots directly into Windows, and if I set it to "Legacy first" it shows me the GRUB menu where I can choose Xubuntu or Windows. Xubuntu boots fine from there, but if I try to boot Windows from GRUB it throws me a fatal error and I have to hard reset.

Here are my questions:

  • Is there a way to convert Xubuntu to UEFI mode? Or Windows to Legacy mode? Note that I can't boot from USB in UEFI mode.

  • Will the computer be harmed if I keep these settings (Xubuntu Legacy mode + Windows UEFI mode)? I can live with that, but I don't know if it's harmful.

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    You just about cannot convert Windows to BIOS. It then requires MBR partitioning or total reformat of drive. But you can convert Ubuntu to UEFI boot. Easiest is just to use Boot-Repair's Advanced mode to uninstall grub/reinstall grub. It uninstalls grub-pc and installs grub-efi-amd64. Best to boot in UEFI mode. Your UEFI should offer both UEFI & BIOS boot of installer.
    – oldfred
    Commented Feb 4, 2016 at 16:50
  • Please see this question and my answer to it. Incidentally, you almost certainly can boot in EFI mode -- just not with the boot media you prepared.
    – Rod Smith
    Commented Feb 5, 2016 at 0:43

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Setup your USB boot media the standard way, and it will be able to boot in either UEFI or legacy mode. It's possible to convert your legacy Ubuntu install to UEFI, (add the /EFI/ubuntu bootloaders, change a few files like fstab, but it's far easier to just reinstall in UEFI mode when you have the proper media. How you can boot the media depends upon the way the media has been set up -- you set it up just for legacy boot, so it can only boot in legacy mode. Be careful when you reinstall Ubuntu -- DO NOT select a "reinstall" grub option, that will wipe the whole disk.

Whatever you do, back-up your important files on the Windows side first. Bad things can happen with a simple typo. Oldfred's boot-repair suggestion should be an easy way to convert the Ubuntu install to UEFI. It is just an extra step to switch modes to access the other OS, I don't know of any harm it could cause.

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  • But is it bad if I keep the current setting? I don't mind having to change from Legacy mode to UEFI to boot Windows (because I won't even use Windows anymore, but all my documents are there).
    – Eduardo Cola
    Commented Feb 4, 2016 at 17:18

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