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I have been using Ubuntu for a few months, and was trying to install Windows 10 as dual-boot for software/games that don't work in Wine.

  • Ubuntu 20.04 is installed on a 120GB Kinston SSD.
  • 1TB Seagate HDD with 250GB NTFS partition where I want to install Windows 10 Pro.
  • HDD has the rest of the space allocated as an NTFS partition with ~650GB of files that I cannot lose or back-up.

I used gdisk in Ubuntu Terminal to convert 250GB partition on HDD to GPT as the entire drive was MBR(as stated above I cannot lose files on other partition). I then booted to the Windows install USB and installed Windows 10 Pro on the 250GB NTFS partition, after which it should automatically reboot into Windows to finish the setup.

In my case it rebooted into Ubuntu, I checked the partition in Ubuntu and confirmed that Windows did get installed. I rebooted and got into my BIOS and noticed that the Windows 10 partition was not in my boot order and not showing up at all, only Ubuntu on my SSD and the Install USB.

I tried reinstalling Windows with the partition this time deleted(250GB as Free Space), and created the partition in in install manager, Windows then made a 16Mb MBR partition in addition to the GTP partition for Windows. The results however were the same. Booting back onto the Install USB I entered Command Prompt from Repair option and used the bootrec /ScanOS command which returned that there are no detected Windows 10 installs.

I have confirmed that I do have EFI Ubuntu installed. Any assistance would be greatly appreciated.

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    I don't think you can have one partition GPT and another MBR. Drive has to be GPT or MBR. Seeing you have data you can't lose, be careful. Also how did you make Windows install USB? – crip659 Jul 2 at 21:50
  • You may have already caused yourself a big problem. If you have managed to convert from MBR to GPT it will have affected the 650GB partition. Check if it’s accessible from Ubuntu. If it is you are lucky and the HDD is still MBR. That could explain why Windows is not booting. If you cannot access the 650GB partition you should do nothing but focus on data recovery. – PonJar Jul 3 at 8:38
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It seems the install actually did work, and the conversion from MBR to GPT as well.

I found this guide Use Grub Customizer to Change Boot Order & Make Windows Default for GRUB customizer and that showed the Windows Boot Loader. I moved Windows Boot Loader up to just below Ubuntu. After rebooting the GRUB menu showed and I was able to boot into Windows. Multiple reboots later I have yet to experience any issues.

Thanks for the replies, All my data drives can be accessed from within Ubuntu and all the data seems to be fine.

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