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Can I install Windows 10 in UEFI mode along side Ubuntu 20.04 in Legacy/BIOS mode?

I understand that similar questions have been asked, but I didn't find my specific case.

I already have Ubuntu 20.04 Legacy/BIOS with GPT disk installed. I want to use gparted on my Ubuntu partition to free up space for Windows 10, and then install it. I am concerned that if I install Windows 10 in Legacy/BIOS mode, it will override my Ubuntu boot and I won't be able to start Ubuntu.

Can I install Windows in UEFI mode, and then when starting PC switching between UEFI and Legacy mode depending which OS I want?

1e87556d-16f8-402c-bce0-e2485e avatar
To add, I only have one hard disk.
1e87556d-16f8-402c-bce0-e2485e avatar
Or should I install Windows 10 in Legacy also, and then use USB Boot Ubuntu to fix boot issues?
cc flag
For a GPT partitioned disk, Windows will have to be installed in UEFI mode, so you will need an EFI partition. Make the EFI partition big enough to hold the shim/grub files in addition to the Windows files (FAT 500M is plenty). After the Windows install, install the grub-efi package to get Ubuntu to boot from UEFI. Might be some minor things to fix up, like adding a /boot/efi mount point and /etc/fstab entry to mount the EFI, but that's only needed for grub/shim updates, nothing to do with booting or running.
1e87556d-16f8-402c-bce0-e2485e avatar
@ubfan1 Sorry, I am new to this and I try to understand. I said that my Ubuntu is GPT (using gdisk I got: Found valid GPT with protective MBR; using GPT), but Ubuntu boots in Legacy, not UEFI. For Windows I don't care If it is MBR, or GPT, I just want it to work along side with Ubuntu. So far I have /dev/sda1 ext4 with mount point /, /dev/sda2 linux swap & /dev/sda3 grub2 core.img with flags: bios_grub. You said to install grub-efi, but I found very little guides(after extensive search) regarding transforming Ubuntu from Legacy to UEFI, so I do not know how to do it.
cc flag
See https://askubuntu.com/questions/1378248/convert-exixting-ubuntu-20-04-disk-to-uefi/1378255#1378255 You should add additional information into your original posting, where formatting options are available. The order is shrink the sda1, add a 500MB FAT EFI partition, add the partition for Windows. Caution, not sure if the Windows install plays nice and will go where you ask to install it. Backup everything first of course.
1e87556d-16f8-402c-bce0-e2485e avatar
@ubfan1 That did help me, thank you. Both of my OSes are now UEFI running.
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