UEFI Secure Boot (Unified Extensible Firmware Interface) is the successor to BIOS (Basic Input Output System) and is used in new 64-bit computers with Windows 8 and above.
For security reasons, UEFI, which is enabled by default, only runs signed bootloaders. Therefore, it is not possible to start the computer from a CD or USB drive, unless the option is disabled.
Due to the fact that the existing GPT partitions require mandatory UEFI, Windows x64 may not boot after disabling secure boot.
1. Hold down the Shift key and click Restart.
2. Click Troubleshoot → Advanced options → Start-up Settings → Restart.
3. Tap the F10 key repeatedly (BIOS setup), before the “Startup Menu” opens.
4. Go to Boot Manager and disable the option Secure Boot.
5. Change the UEFI boot order according to the medium you want to use to start the computer.
6. Save the changes and reboot.