When you dual-boot Windows 10 with an earlier versions of Windows like Windows 8.1, then you have 30 seconds to choose an operating system you wish to boot in before the default operating system runs automatically. Part 1: How to Change Default Operating System to Run at Startup (Boot Manager) in Windows 10

Part 2: How to Use Windows Boot Genius to Fix Windows Boot Manager Errors and Other Windows Boot Issues.

Part 1: How to Change Default Operating System to Run at Startup (Boot Manager) in Windows 10.Windows Boot Genius is the first PC utility aiming to diagnose and troubleshoot Windows boot manager problems and other Windows won't boot issues like booting failure, crash, freeze, black screen of death, or blue screen of death. Nevertheless, if this does not do the trick, a professional Windows boot fix tool is a good choice. Sometimes a simple Windows restart can fix the boot manager issues. The possible reasons for BOOTMGR errors include corrupt and misconfigured files, hard drive and operating system upgrade issues, and outdated BIOS, damaged or loose hard drive interface cables, or your PC is trying to boot from a hard drive or flash drive that is not properly configured to be booted from. Sometimes it displays 'Couldn't find BOOTMGR', shortly after the computer is turned on. 'BOOTMGR is missing' is one of the common startup errors in Windows. And also you can set the default boot entry so that the operating system will load what you prefer automatically. You can select which operating system you would like the Windows to boot in the Boot menu.

BOOTMGR is used to start your Windows 10, Windows 8, Windows 7, or Windows Vista operating system startup.

Officially, Windows Boot Manager (BOOTMGR) is a small piece of software, called a boot manager.

Windows boot manager missing