Comment 2 for bug 1906256

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oldfred (oldfred) wrote :

Windows requires MBR(msdos) partitioning for BIOS boot.
Ubuntu will let you install in UEFI mode to MBR drive, but probably should not as Windows wants boot partition to be its partition with boot files (system), but UEFI wants boot flag on ESP - efi system partition.
UEFI highly recommends gpt partitioning for UEFI, and Windows only installs in UEFI mode to gpt drives. Normal reinstall in different mode will erase drive. So if you attempt to reinstall Windows in UEFI mode, it will convert drive to gpt & totally erase drive. Or have good backups.

Either reinstall Windows in UEFI mode since you have UEFI hardware. Microsoft has required vendors to install in UEFI mode to gpt partitioned drives since 2012 and release of Windows 8.

Or only install Ubuntu in 40 year old BIOS/MBR configuration.

How you boot install media, UEFI or BIOS is then how it installs.