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I recently installed Windows 8.1 from a USB drive. In order to boot from the USB drive, I had to disable secure boot in the BIOS and switched to Legacy boot mode (from UEFI boot mode).

After the installation finishes, I no longer need to boot from the USB drive so I want to switch back to secure boot (UEFI boot mode) but then Windows wouldn't boot.

What did I do wrong? Do I need to install Windows again?

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If you want to boot windows through UEFI, you must boot windows installer through UEFI. I'm not sure about windows 8.x, but for windows 7 installation media you had to do some manual fiddling around - last paragraph on https://www.thomas-krenn.com/en/wiki/Creating_Windows_UEFI_Boot-Stick_in_Windows

This was probably why you couln't start the installation through UEFI.

Righ now you have a BIOS installation of windows and no way to change it to UEFI without reinstalling, sorry.

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The main important part of UEFI is you should use GPT partition and 64bit OS. But remember do not convert External drive to GPT.

The boot partition (usually 100 MB) must be in FAT32

Depend on motherboard, some motherboard read the GPT boot automatically without doing anything after you convert it from MBR.

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