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.
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.
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.

The following method allows you to download Mac OS X Lion, Mountain Lion, and Mavericks. Start your Mac holding down Command + R. Prepare a clean external drive (at least 10 GB of storage). Within OS X Utilities, choose Reinstall OS X. Oct 29, 2019 Therefore there are very few documentations regarding clean installation for Mac OS relative to Windows Reinstallation. In this tutorial, I would try to cover up as much detail I could to help you Reinstall a Clean Mac OS on to your Macbook or iMac. I recommend you to use 16GB or higher USB drive. Sep 06, 2019 Use a Mac OS X installation Disc. If you’re unable to use Internet Recovery Mode or create a bootable USB installer, you can still use a Mac OS X installation disc. These discs are available for OS X Snow Leopard, OS X Lion, and OS X Mountain Lion. If your Mac is from 2012 or earlier, there was an installation disc in the original box.

How to boot a Mac from USB media

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

Insert the USB boot media into an open USB slot.
Press the Power button to turn on your Mac (or Restart your Mac if it’s already on).
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.
Using either the pointer or arrow keys on the keyboard, select the USB drive you wish to boot from.
Once selected, either hit the Return key or double-click your selection. The machine will start to boot from the USB drive.

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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:

Press the Power button for your computer.
During the initial startup screen, press ESC, F1, F2, F8 or F10. (Depending on the company that created your version of BIOS, a menu may appear.)
When you choose to enter BIOS Setup, the setup utility page will appear.
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.
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
Save the change and then exit the BIOS Setup.
The computer will restart using the new settings, booting from your USB drive.
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.

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Linux USB Boot Process

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

Confirm the BIOS boot sequence lists the USB drive first, or make that change as needed.
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).
The installer boot menu will load, where you will select Run Ubuntu from this USB.
Ubuntu will launch and you can begin to working in the system – setting preferences, reconfiguring the system as needed, or running any diagnostic tools.

Creating USB boot media

Regardless of the operating system you are using, booting your machine from USB media does not need to be difficult. A general understanding how your system loads can provide the basics needed to understand what is going on when you use boot media.
Creating USB boot media doesn’t need to be difficult either, although there are several options to consider.
For Mac users, we recommend visiting Apple’s support page on USB boot media. It provides guidance that is specific to the iteration of OS X you are running (i.e. Sierra, High Sierra, Yosemite, etc.) to help you get the boot version you need.
Windows and Linux users might consider Acronis Disk Director 12, which includes an intuitive Boot Media Builder that streamlines the process and offers tremendous flexibility for the type and kind of boot media you can create, including WinPE media.

Those who still rely on older Mac OS X operating systems, know these still are going strong. There are so many things you can do with past OS X — like Lion, Mavericks or Yosemite — each one of these operates flawlessly. So if you’re looking to give your Mac a fresh start with just a classic Mac OS X and nothing else, to clean install Mac OS from USB is the best option for you.

To clean install Mac OS from USB, there are three things you should do. First, you should clean up your Mac from the old baggage — system files that have piled up over time. Once it’s all cleaned up, the next you should do is backup your newly cleaned Mac. And finally, after doing a backup, you can clean install OS X of your choice from USB.

We’ve got the steps outlined below, so let’s get to it.

Step 1: Clean system junk on your current Mac OS X

Whatever OS X you are currently on, system junk and outdated cache files are a potential source for software conflicts. Having cleaned them up, you may discover around 35 GB of newly created space previously taken by junk. What's more, old caches may interfere with your new Mac OS X installation, so this step is pretty essential.

Cleaning can be done either manually or using a dedicated app. If you don’t want to spend the next 20 minutes on repetitive clicking and typing, jump right to the automatic solution further down.

The manual way to remove old user caches:

Click on a Finder window and choose “Go to Folder” in the Go menu.
Type in ~/Library/Caches and click Enter to go open the folder.
Go through each of the folders and remove their contents.

Note: Do not remove the folders themselves, just their insides.

Next off, do the same steps, but now replace...

~/Library/Caches with… /Library/Caches

Automatic solution for system junk cleanup

There are many nice apps that offer a quick Mac clean up. Among those, CleanMyMac 3 is our favorite as it looks most user-friendly of

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