Hi everyone sorry for my english. Is it possible to install windows 7 over windows 10 (because i am not liking it and it's lagging a lot) without losing ubuntu? I installed ubuntu with dual boot along with windows 10
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i think your question is almost installing windows after installing Ubuntu. also i guess BIOS mode and UEFI GPT partitioning MBR partitioning will make situations critical in your situation.. – PRATAP Feb 02 '19 at 16:15
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https://askubuntu.com/q/1042813/739431 may help – PRATAP Feb 02 '19 at 16:19
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Related: How can I install Windows after I've installed Ubuntu? – wjandrea Feb 02 '19 at 17:43
1 Answers
Ignoring the many reasons for the poor performance of your Windows 10, the short answer would be: "Yes. That should be possible."
Windows and and Ubuntu are installed on each their own partition. Get our your Windows 7 installation, select the Windows 10 partition when installing and go. Your Ubuntu partition will be unaffected by this. You could even install on a third partition, if you wanted.
Running the windows installation will delete your grub, and hence your chance to select OS on boot. To fix this you will have to reinstall the grub in Ubuntu. To do this you will have to boot from a live CD or USB-drive with eg. Ubuntu on it. Load the live OS from the drive and use sudo grub-install /dev/XXX XXX is the install directory. Most likely sdb. eg: "/dev/dsb" More information on this: https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows
Note: You might be able to do a simple downgrade, depending on how you upgraded from 7 to 10. There are various guides for this, try googling "Downgrade from Windows 10 to 7". You will still have to reinstall grub whether reinstalling or downgrading as Windows overwrites the bootloader.

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You didn't mention how to restore grub, as windows 7 installation will overwrite it. – LeonidMew Feb 02 '19 at 20:48
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Thank you @LeonidMew. I indeed forgot that. I updated the post with information regarding grub. – Amiga85 Feb 04 '19 at 08:15