As far as I have heard, dual booting (Ubuntu and Windows) in UEFI isn't very clean, and hence people prefer CSM when doing so. But, in case my laptop has Windows pre-installed in UEFI, I need to reinstall it to get it in CSM.

Now, my laptop's boot menu has an option of UEFI and CSM. What could this option mean? Does it mean I can dual boot without any hassles?

  • Your Windows Product key is in UEFI and only for your OEM or vendor version, so if you want to install in BIOS boot mode, you must purchase a new Windows license with a product key you can input. Depending on brand/model just install Ubuntu in UEFI boot mode. Seel LiveWireBT's post. – oldfred Dec 24 '14 at 17:09

Each firmware can be different, but generally this option means that your firmware will first try to boot in UEFI mode (it will search for .efi files corresponding to your UEFI entries), then if it fails to boot in UEFI mode it will try to boot in Legacy mode.

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dual boot windows 8.1 and Ubuntu 14-10

  1. Install Windows 8.1
  2. Install Ubuntu
  3. Restart
  4. Your computer will no show you the option to select the operating system that you want to boot. Don't worry, boot into Ubuntu
  5. Open a terminal
  6. Run de following command update-grub

Enjoy your dual boot!

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Dual-booting Windows and Linux in EFI mode can be done in a perfectly "clean" way. In fact, it's got some advantages over BIOS/CSM/legacy mode. The main troubles with EFI-mode dual-boot setups are with buggy firmware and with a lack of user understanding of the issues. The firmware bugs are being slowly resolved, but there are still too many problems. (Sometimes upgrading the firmware will help.) User understanding can be acquired by reading appropriate sites. Two I recommend are:

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As far as I have heard, dual booting (Ubuntu and Windows) in UEFI isn't very clean

I need to reinstall it to get it in CSM.

This is not true.

As @RodSmith already said, there are firmware bugs or seemingly deliberate faulty implementations. The device manufacturers are to blame for this, not Microsoft! (If you're blaming Microsoft and do nothing against the bad firmware from device manufacturers things will remain as they currently are.)

In case of these issues I was always able to boot from the UEFI loader for removable media.

Also you don't have to reinstall an operating system just for changing the way it boots. You can setup a BIOS Boot Partition on an existing Windows 8 installation or you install GRUB to a GRUB BIOS Boot Partition to enable legacy booting. I wrote what can be done from a perspective of legacy booting (MBR) and Windows 8 (GPT):

The topic is actually trivial from a consumer perspective. As it should be.

Where does the confusion come from?

It's the golden hammer.

While I understand that people would want a tool like boot-repair, I also want them to read the collection of shell scripts it consists of and tell me that this is well written and documented code (which coding style?) before demanding inclusion in Ubuntu and on the live media, where maintainers would have to maintain the code and possibly improve the existing resolution strategies it uses.

Of course, I know who the author is and it's nothing personal.

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