The boot order tells the machine which devices to search for the software needed to launch the computer and the priority each device in that search. That’s because the BIOS settings include the boot sequence the machine follows when starting up.
Starting your PC using USB rescue media is not difficult, although it first requires an adjustment in the BIOS (Basic Input Output System). The Startup Manager only lists drives that include bootable content. NOTE: Have multiple USB devices connected to your Mac? Don’t worry.
The machine will start to boot from the USB drive. Once selected, either hit the Return key or double-click your selection.
Using either the pointer or arrow keys on the keyboard, select the USB drive you wish to boot from.The utility will look for any available drives that include bootable content. Once the Startup Manager screen appears, release the Option key. Holding that key gives you access to OS X’s Startup Manager. When you hear the startup chime, press and hold the Option key.
Press the Power button to turn on your Mac (or Restart your Mac if it’s already on).Insert the USB boot media into an open USB slot.Getting your Mac to load from a USB drive is fairly straightforward. So if your system is unstable, you need to run a diagnostic tool on the hard drive, or you just want to load a Linux desktop just to see what it’s all about, let’s look at how you start your machine using rescue USB boot media. How you start that rescue media can vary depending on the operating system you are using, but there are a few general guidelines that can help get your machine started, regardless of the OS you prefer. As a result, booting from USB media is becoming the standard. If it says “Safe” then your computer is in safe mode.And while users historically would pop a boot media disk into their DVD or CD drive, many computers no longer come with optical disk drives. This will launch the System Information app. While pressing the Option key, choose Apple menu > System Information.The other method is to use System Information.Your lock / login screen will say “safe boot” in red in the menu bar:.So you may suspect that perhaps your Mac is not in Safe Mode. When your computer is in Safe Mode, your computer will appear the same. If you cannot exit safe mode, please see this article. Restart your Mac normally without pressing the Shift key.Here is how you can restore the system to its normal mode: When you see the login screen, release the Shift key.Keep holding the Shift key, wait for the login screen.Turn on your Mac and immediately after hearing the startup tone, press and hold the Shift key.When you see this screen, release the power button.