While fans of different operating systems might debate which one is the most stable, reliable, flexible or user-friendly, there’s one inarguable point: Every machine – regardless of the Operating System – can run into issues.

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.  As a result, booting from USB media is becoming the standard.  However, the method to booth a DVD or CD in your DVD or CD-Rom drive is the same as booting from the USB Flash Drive.  The difference being that you would select your DVD Drive as the main boot drive.

How you install Linux or 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.

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 to boot a Mac from USB media

Getting your Mac to load from a USB drive is fairly straightforward.

1. Insert the USB boot media into an open USB slot.
2. Press the Power button to turn on your Mac (or Restart your Mac if it’s already on).
3. When you hear the startup chime, press and hold the Option key. Holding that key gives you access to OS X’s Startup Manager. Once the Startup Manager screen appears, release the Option key. The utility will look for any available drives that include bootable content.
4. Using either the pointer or arrow keys on the keyboard, select the USB drive you wish to boot from.
5. Once selected, either hit the Return key or double-click your selection. The machine will start to boot from the USB drive.

NOTE: Have multiple USB devices connected to your Mac? Don’t worry. The Startup Manager only lists drives that include bootable content.

Boot from USB: Windows

Starting your PC using USB rescue media is not difficult, although it first requires an adjustment in the BIOS (Basic Input Output System). That’s because the BIOS settings include the boot sequence the machine follows when starting up. 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.

If you’re booting from USB media, you must change the BIOS boot order so the USB device is listed first. Otherwise the computer will load from the hard drive as normal.

Start by plugging the thumb drive into a USB port. Then to change the BIOS boot sequence:

1. Press the Power button for your computer.
2. During the initial startup screen, press ESC, F1, F2, F8, F10 or F12. (Depending on the company that created your version of BIOS, a menu may appear.) If you're not sure, most computers will show which key to press for the BIOS or the Boot menu right during booting when you power on your system.
3. When you choose to enter BIOS Setup, the setup utility page will appear.
4. Using the arrow keys on your keyboard, select the BOOT tab. All of the available system devices will be displayed in order of their boot priority. You can reorder the devices here.
5. Move USB to be first in the boot sequence.
   
   NOTE: If you cannot find USB or Removable Devices among the device options, your BIOS may list it under Hard Drive Devices. In that case, you’ll need to:
   • Move Hard Drive Devices to the top
   • Expand to show all hard drive device options
   • Move USB device to the top of that hard drive list
6. Save the change and then exit the BIOS Setup.
7. The computer will restart using the new settings, booting from your USB drive.
8. Stay alert! Depending on your BIOS, you may be prompted with a message to Press any key to boot from external device and you will only have a few seconds to respond. If you do nothing, your computer will go to the next device in the boot sequence list, which will likely be your hard drive.

In the future, your computer will first check the USB port for boot media when starting up. That won’t be a problem, since the BIOS will move to the next device in the boot sequence ... unless you keep the boot media in the USB port. Then the system will launch from that device every time.

Linux USB Boot Process

To boot Ubuntu from USB media, the process is very similar to the Windows instructions above.

1. Confirm the BIOS boot sequence lists the USB drive first, or make that change as needed.
2. After the USB flash drive is inserted into the USB port, press the Power button for your machine (or Restart if the computer is running).
3. The installer boot menu will load, where you will select Run Ubuntu from this USB.
4. Ubuntu will launch and you can begin to working in the system – setting preferences, reconfiguring the system as needed, or running any diagnostic tools.