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I bought a dell laptop with pre-existing ubuntu 14 installation. I use both ubuntu and windows on my main machine. when I tried to install windows alongside ubuntu 14 and after I formatted a space from the main drive for it, it told me it can't be installed on this disk because it's a gpt type disk, and the only solution is to delete the disk (in case of booting from usb, you can't boot to usb in uefi mode in this laptop). then I tried to enter ubuntu 14 again, but it wouldn't boot. with this message appearing: `error failure reading sector from 'hd1' so I searched for this message but all the solutions was about a live cd that won't boot. so after that I decided to follow this guide. I partitioned the disk as mentioned there but in uefi mode. then as mentioned I tried to format the windows partition in windows setup as a ntfs drive. the setup told me the same earlier message about gpt type disk. then I tried to open ubuntu 16 again and it booted into emergency mode.

I'm writing this post as I'm installing windows 10 after deleting the whole disk. this question is about knowing the exact reasons and for whom who might encounter the same problem to help him dodging this problem. and if there're some advices on installing ubuntu 16 next.

laptop is dell inspiron 5559 i5-6200U

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mohRamadan
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The error message you're seeing, while technically accurate, tends to lead people on wild goose chases. The problem is not the partition table type; it's the boot mode. Specifically, your existing Ubuntu installation was in EFI mode and used the GUID Partition Table (GPT), but you booted your Windows installation medium in BIOS/CSM/legacy mode using a feature called the Compatibility Support Module (CSM). When booted in BIOS mode, the Windows installer insists on installing to an MBR disk, not to a GPT disk.

The solution is to ensure that your Windows installation medium supports EFI-mode booting, and boot it in that mode, either by disabling the CSM or by using firmware-specific features to boot in EFI mode. You can probably disable the CSM using your firmware setup utility, but the details of how to do this vary greatly from one EFI to another. Ensuring that the Windows installation medium supports EFI-mode booting requires using the right tool to prepare the boot medium, and I'm not an expert on that subject for Windows boot media. You may want to ask on a Windows forum about that.

I have written a web page about this subject you may want to read; however, my page is written from the perspective of a system with Windows pre-installed and the user wants to install Linux, not the other way around. The principles involved are the same, but some important details (like preparing the OS installation boot medium) differ. (I'm not 100% positive, but I think Rufus will write either Linux or Windows installation images, though.)

Note also that Windows will set its boot loader as the default after it installs. Ubuntu is smarter about detecting Windows and providing a Windows boot option. This is why most instructions suggest installing Windows first and Ubuntu second. If you're re-installing everything from scratch, this is the way to go. If your Ubuntu installation still boots, though, you can install Windows second and then use any of several tools to adjust the boot order after Windows is installed. The third-party EasyUEFI is likely to be the best choice for this task. You'll then need to run sudo update-grub to have GRUB detect Windows and offer it as an option.

Rod Smith
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