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Hardware

What is boot order?

Boot order is the priority list in BIOS/UEFI setup that decides which device a computer tries to start from first, such as an SSD, USB drive or network.

What is boot order?

After the power-on self-test (POST), the firmware has to find something to start. It works down the boot order, also called boot priority or boot sequence, and hands control to the first entry that has a working boot loader. If an entry has nothing bootable, the firmware moves on to the next one. The order is saved with the other firmware settings and is changed in the BIOS/UEFI setup utility.

Boot options vs. boot order

  • A boot option is one entry the firmware can start from: Windows Boot Manager, a specific SSD, a USB flash drive, an optical drive, or a network (PXE) entry.
  • The boot order is the ranking of those entries.
  • On UEFI systems, an installed operating system adds its own entry, such as Windows Boot Manager or a Linux boot loader like GRUB, that points to a boot file on the EFI System Partition rather than just to a disk.

Typical boot devices are the internal SATA or M.2 NVMe drive, USB flash drives for installers and repair tools, optical drives on older systems, and network boot for imaging in business and school environments.

Changing the boot order

  1. Restart and press the setup key during POST. Common keys are Del, F2, F1, F10, or Esc, depending on the manufacturer.
  2. Open the Boot section.
  3. Move the device you want to the top, or disable entries that should never be tried.
  4. Save and exit, often with F10.

To start from a USB installer once, use the one-time boot menu instead of changing the saved order. The key varies by vendor: F12 on Dell and many other systems, Esc and then F9 on HP business systems, and F8 or F11 on many desktop motherboards. Fast startup can make the setup key hard to catch. From Windows, hold Shift while selecting Restart, then choose Troubleshoot > Advanced options > UEFI Firmware Settings to restart straight into setup.

Troubleshooting with boot order

  • Bootable device not found, No boot device, or Operating system not found: first check whether the system drive appears in firmware at all. If it does, make sure it (or Windows Boot Manager) is first and that no USB stick or disc is ahead of it. If the drive is missing from firmware, suspect the cable, the drive, or a disabled SATA or M.2 port.
  • PC keeps starting the Windows installer after setup finished: the installer USB is still plugged in and ranked above the internal drive. Remove it or move the internal drive up.
  • Bootable USB not listed: USB boot may be disabled, Secure Boot may block the tool, the stick may be built for legacy BIOS boot while the system is set to UEFI only (or the reverse), or the stick is not actually bootable.
  • Long pause and a network boot message before Windows: network boot is ranked above the internal drive, or the drive was not detected and the firmware fell through to the network.
  • Boot order resets after the PC is unplugged, and the clock is wrong: the CMOS battery is failing, so firmware settings are not kept.

Boot order and security

Anyone who can boot a USB tool can bypass Windows sign-in and read an unencrypted drive. On managed machines, technicians put the internal drive first, disable or restrict USB and network boot, and lock setup with a BIOS password so the order cannot be changed. Full-disk encryption protects the data even if someone does boot another OS.

Boot order on the CompTIA A+ exam

Core 1 objective 3.5 lists boot options under BIOS/UEFI settings, next to USB permissions, Secure Boot, boot password, and BIOS password. Objective 5.2 lists bootable device not found as a drive symptom. The exam expects you to check boot order and drive detection before assuming the drive has failed. Installing an operating system from USB or the network, covered in Core 2, also starts with selecting the right boot device.

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Related terms

BIOS/UEFI

BIOS (Basic Input/Output System) and UEFI (Unified Extensible Firmware Interface) are types of firmware stored on a motherboard that initialize hardware during boot and provide runtime services to the operating system. UEFI is the modern successor to the legacy BIOS.

bootloader

A bootloader is a specialized program that initializes hardware and loads the operating system kernel into memory when a computer starts up, serving as the critical bridge between firmware and the fully functional OS.

CSM

CSM (Compatibility Support Module) is a UEFI firmware feature that emulates legacy BIOS so older operating systems, MBR disks and boot media can start.

PXE

PXE (Preboot eXecution Environment) is a standard that lets a computer boot from a server over the network through its network card, without a local drive.

Secure Boot

Secure Boot is a security feature that ensures a computer's firmware and software are verified as authentic and trusted upon system startup, preventing malware from loading during the boot process.

USB flash drive

A USB flash drive is a small removable storage device that stores data on NAND flash memory and plugs into a USB port. It is also called a thumb drive.

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