After the download is done, it will install the fresh version of the the macOS operating system and you can carry on with the set-up. Install macOS Using a USB Bootable Drive. Using a USB Bootable Drive to reinstall macOS is very handy as you don’t have to download the entire macOS operating system again. As I've said before, slowing down is not an option for me. At one point, I realized that there were plenty of files on my Mac that I didn't need any more, but too many to delete individually. Select the Recovery HD and click on the arrow button below the icon. When the Utilities Menu appears select Disk Utility and click on the Continue button. Follow the instructions in the below link for the OS X version you wish to reinstall. Install or Reinstall Mavericks or Mountain Lion from Scratch. I just did a fresh install of Mountain Lion OS X version 10.8 onto a USB. The install takes 13.9GB of the USB after installation.
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Whenever a new version of OS X is released, I use it as an excuse to rejuvenate my system by doing a clean install. This has become harder as Apple has started to release their OS through the App Store as an update only. Most guides out there to clean install the OS involve some command line gymnastics and tricks involving downloading the update and not running it etc. There is a much easier way using only standard options. The catch, it takes twice as long.
The strategy is to upgrade the OS on your computer as intended by Apple through the App Store. Use the new Recovery Tool from your updated Mac to install Yosemite on a USB drive. This will allow you to then boot from the USB drive and make it possible for you to unmount your computer’s internal drive, format it and install Yosemite on it from the USB drive. You can do all of this using standard options. No tricks needed!
Yes, we will take a step back to take two steps forward. Simply go ahead and upgrade your Mac to Yosemite through the App Store. This should be a pretty standard and painless process. Hopefully it should go by the book.
Once the installation is complete and you’re looking at your new desktop, move to the next step.
Let the installer install the OS on the USB Drive, respond to all the prompts as needed and wait till you get to a fresh Yosemite desktop.
Important: Your computer is currently running off the USB drive, do not remove it.
Running Recovery from the USB Drive will now actually allow you for format your internal hard drive.
Let the installer install the OS on the hard drive and respond to all the prompts as needed. You will eventually end up in a brand spanking new and clean Yosemite desktop, running off your hard disk. You can now get rid of your USB drive and enjoy your brand new, cleanly installed OS X Yosemite.