When you’re troubleshooting your computer or installing a new operating system, you may need to “boot from a disc” or USB device (such as an external hard drive or flash drive). Here’s what that means — and how to do it.
When you press the power button on your computer, it boots up from its internal hard drive, where your operating system (usually Windows or macOS) is stored.
But sometimes you need to boot into something different. Maybe you need to boot from your Windows installation disc to reinstall Windows, or perhaps you need to boot from a system rescue disc to solve a problem with your machine. You may simply want to run a different operating system on your computer.
On a Windows PC
Every Windows PC is different. Some PCs are already set up to search for a USB option before defaulting to the operating system on the internal hard disk. But once you get into your system’s boot menu, you should be able to find what you’re looking for.
Here’s how to boot from a CD or USB drive on a PC:
Restart your computer
Restart your computer and wait for that first screen to pop up. Often, it’ll say something like “Press F12 to Choose Boot Device” somewhere on the screen — the requested function key could vary depending on your machine. Press that key now.
Wait a second
Give it a moment to continue booting, and you should see a menu pop up with a list of choices on it. Highlight your preferred CD or USB drive and press Enter.
Alternatively, you can set your computer to always check for a bootable CD or USB drive and change the boot order. That way, when you have an external disc inserted, your machine will boot from it automatically, and when you don’t, it’ll head into your regular operating system. To set this up, follow these instructions.
Restart your computer and watch for the first screen that shows up when you boot. You should see a message like “Press DEL to enter SETUP” or something similar (the requested key may vary depending on your machine). Press the noted key on your keyboard and wait for setup to start.
Select ‘Boot Device’
You should see a new screen pop up, called your BIOS. This is where you set a lot of low-level settings for your computer. BIOS menus are configured differently across machines, but you should see instructions for how to navigate yours (that is, which keystrokes to use) somewhere in the window.
Using those instructions, look through your menu options (be careful not to change anything else) for a setting called “Boot Device”, “Boot Order”, or something similar. Select that option.
Choose the right drive
From the menu that pops up, choose your computer’s disc drive and press Enter. If you’re trying to boot from a USB drive, choose that option. It might be listed on its own or under a sub-menu such as “Removable Devices”. Press Enter.
Depending on your BIOS, you may need to use the Page Up and Page Down or +/- keys to move your selection to the top of a list instead. Again, use the directions listed on your specific machine.
Exit the BIOS
Follow instructions to exit out of your BIOS and save your changes. Usually, this option is under “Exit” or “Save and Exit” on the main menu or available via a keyboard shortcut listed somewhere on your screen. Depending on your BIOS, you may have to confirm your selection again.
Your computer should restart automatically. Make sure your CD or USB drive is in your computer. If you’re prompted to “Press any key to boot from external device”, do so. Your computer should boot into the CD or USB drive instead of your normal operating system.
From there, you can follow the on-screen instructions to install Windows, troubleshoot issues, or do whatever else it is you need to do.
On a Mac
Booting from a USB or CD is very, very easy on a Mac. One way to do this is to open System Preferences > Startup Disk. You’ll see your built-in hard disk as well as any compatible operating systems and external drives.
Click the lock icon at the bottom-left corner of the window, enter your admin password, select the startup disk you want to boot from, and hit Restart. This method will save your startup disk preference until you go through this process again to choose a different option.
To do a one-off boot from a USB, you’ll use the Startup Manager instead. Here’s how it works:
Reboot your computer
Reboot your computer. Press and hold the Option key as soon as your machine restarts. Release when the Startup Manager window opens (or when you’re asked to put in your firmware password).
Choose the right drive
You should see icons for your available drives. On the right, you’ll find your external (CD or USB) drive with its name listed underneath. Use your arrow keys or your mouse to highlight that drive, then press Return or click on the arrow below it. (If you press and hold Control at the same time, your computer will save this as your preference — similar to the process outlined above.)
From there, your computer will boot into the CD or USB drive instead of OS X, and you can do whatever it is you need to do by following the on-screen instructions.
This story was originally published on 23 March 2013 and was updated on 29 September 2019 to provide more thorough and current information.