I have Windows 8 installed on my system. To install Ubuntu from a bootable USB, I was not getting the boot from USB option. To get that I found a solution that I have to switch to Legacy Boot. So I did that and then installed Ubuntu 16.04. After a successful installation, when I restarted the system I did not get the grub boot loader menu to choose the OS. Instead it boots directly to Ubuntu. Now if I change the boot option in Boot Manager to UEFI, then it boots directly to Windows 8. So should I have to install both OSs in the same Legacy mode to get the Boot Menu at startup?

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    Yes, you should install both in UEFI mode (Windows already is). Some notebooks, however, need additional steps to use/boot from external USB drives but at the end of the day it's obviously preferable to address those issues and have everything installed as it should be than to use the legacy workaround. – user692175 Nov 3 '17 at 6:53

Enabling BIOS/CSM/legacy support was a mistake. For all the gory details, see my Web page on the subject.

Instead, you should have looked into overcoming your problems booting the installation medium. Chances are it was improperly prepared -- some tools don't copy the EFI-mode boot loader from the .iso file to the USB flash drive; and sometimes there are quirky incompatibilities between specific tools and specific computers. The page I just referenced includes a section on this subject, so read it for details. The short version is that Rufus and the standard Linux tool dd generally do the best job, in my experience.

Thus, one way to fix your problem is to disable the CSM in your firmware settings, re-create your installation medium, and re-install Ubuntu. This approach is overkill from a technical perspective, but is likely to be the easiest solution, given that you haven't yet used Ubuntu much. If you're averse to an "overkill" solution or if you've spent much time customizing your settings or creating Ubuntu-specific data, though, you may prefer to install an EFI boot loader for Linux to replace the BIOS-mode boot loader you've got now. The two ways to do this that are likely to be easiest are:

  • Boot Repair -- The Boot Repair tool can re-install GRUB to your computer. You'll have to run this tool from an EFI-mode emergency system, though; running it from your current BIOS-bootable installation will be completely pointless. Thus, you'll need to disable your CSM, or at least learn how to control your boot mode.
  • rEFInd -- You can install my rEFInd boot manager, which is an EFI-mode boot manager. You can install it manually from Windows, which is likely to be awkward; or you can download the USB flash drive version of rEFInd, use it to boot Ubuntu in EFI mode, and then install the Debian package or PPA to install it permanently on your hard disk. One caveat is that, if Secure Boot is active, you'll need to disable it, and if you want to re-enable it, you'll need to jump through some hoops.
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    Thanks. I had found solution without enabling the Legacy. My windows 10 was on UEFI already. But I was not getting the boot from USB option. for that I have to off the secure boot option in UEFI. after that I am able to boot and install ubuntu with UEFI. and also able to get boot loader to choose os at startup – nirmal patel Nov 8 '17 at 13:33

Just Type a command sudo update-grub it works I had tried that. If you want can refer this video link https://www.youtube.com/watch?v=j4n_8PlNt_U

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    Typing sudo update-grub from a BIOS-mode Ubuntu when Windows is installed in EFI mode, as is the case for nirmal patel, will do no good whatsoever. It's best if both OSes are installed in the same boot mode (BIOS vs. EFI), as managing cross-boot-mode OS installations is awkward at best, and sometimes impossible. – Rod Smith Nov 3 '17 at 13:48

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