How to "Nuke and Pave" OS X

I have been having some really frustrating issues with my iMac for a while now. About half the time when I sit down at the computer and open up Safari it takes about 60 seconds for a website to load, yet if I open the same site on my iPad (which is using the same wireless network) it loads the page in less than a second. There are also times on my iMac where I have no internet connection at all despite the wireless network being up an running strong. Something was defintely wrong with my wireless connection, but was it hardware or software related. If I’ve learned one thing from dealing with Apple Care it is everything you can to troubleshoot the problem before calling them. So, to rule out any potential software problems I decided to go ahead and do a “nuke and pave” of OS X, hoping that would solve the problem. I was suprised that there wasn’t a lack of really good step by step instructions on how to wipe the hard drive and do a complete re-install of the Operating System (OS). So I figured I would take notes on what I did and post the steps I used to do a complete erase and re-install of Mac OS X.

Do I Really Need to do This?

This is a question you have to ask yourself before you take this drastic step. Most probelms can be fixed without the need to completely re-install the OS. In my case I had already exausted all the “usual suspects”, plus I really wanted to get the partition of Windows 8 off my mac. (can you blame me?) There is nothing wrong with installing a new version (upgrade) of the OS right on top of the existing OS. I have been doing this for years and haven’t had any problems (my current problem cropped up long after I installed Mountain Lion right on top of Lion). So seriously, think long and hard before you go through the pain of starting completely over with a fresh install.


There are several things you need to do before you do a nuke and pave: Backup (do as many of the following as possible):

  • Physical on site
    • Manual via external hard drive
    • Automatic via Time Machine
  • Cloud or virtual backups
    • Cloud services like BackBlaze and CrashPlan
    • Backing up to a friend/family offsite computer
    • DropBox (yes this counts if you have it running on a 2nd PC somewhere else)

App Inventory & Keys
Be sure to have a list of all the apps you must re-install (unless you plan to restore from a Time Machine backup that was backing up your applications and their corresponding application support data located in the following folder:

~/Library/Application Support

Since I wasn’t sure if my 1st attempt at re-installing the OS was going to fix the problem I was prepared to re-install both ways if needed:

  • Restore from a Time Machine backup (really easy)
  • Manually re-install all applications (cleaner but very time consuming)

I also found a really handy checklist created by Asian Efficiency that lists all the things you should think about before reformatting your Mac. Another thing I did was take screen shots of my LaunchPad apps (or the Applications folder) so I had a list of all my applications. Buying from the Mac App Store makes this much easier, but there are always a few apps that I have to buy outside the Mac App Store and its nice to have a complete list. One last thing to consider are the apps that have settings or profiles that you don’t want to lose, like Hazel and Omni Focus. I had Omni Focus on multiple computers so that wasn’t an issue for me, but for Hazel I went ahead and exported all of my rules out to Dropbox just in case I had to do a complete re-install of all my apps from scratch instead of from a Time Machine backup.

Creating an Install Disk

The next step that is required in order to completely “nuke” or erase the hard drive and re-install the operating system is to create a boot disk. This is a DVD, external drive or (preferably) a USB thumb drive because that is the media that produces the fastest boot (booting off of a DVD is incredibly slow. You are going to need a minimum of 4-5 Gb of free space on whatever you choose to create a boot drive for Mountain Lion. I used an 8Gb USB drive. You can manually create a bootable disk using the built-in Disk Utility program on your Mac, but an even easier way to go is to use the free utility Lion Disk Maker. Once you install Lion Disk Maker it walks you through the couple of steps required to create a Mountain Lion boot disk.

Erase and Re-Install

When you are finally ready to pull the trigger just simply follow the steps below. One really important point to remember when going through these steps is to remember that what you are doing is booting off of another disk other than your computer’s regular hard drive so that you can full erase and reformat your computer’s hard drive. You can’t erase and reformat a drive that you are using to boot off of (hence the need for creating a boot drive). The step for erasing and re-installing the OS are as follows:

  1. Choose Apple menu > Restart (in other words restart your Mac) and as soon as your Mac begins to re-boot press and hold the Command and R keys at the same time. This command is telling your Mac to boot from a disk other than your normal boot disk and will prompt you to choose a disk to boot from.
  2. Choose a drive or disk to boot from (choose the media you used to create your Mountain Lion boot disk)
  3. Select the Disk Utility program from the list of options presented
  4. Select your normal boot disk from the list of drives attached to the system and click the “erase” tab
  5. From the format pop-up menu choose “Mac OS Extended (Journaled)”
  6. Enter a name for your new disk and then click “Erase”...this is REALLY GOING TO ERASE YOUR HARD DRIVE NOW
  7. After your hard drive and has been erased and reformated go ahead and exit Disk Utility
  8. From the main screen you are returned to when exiting Disk Utility choose the “Reinstall OS X” option and you will be guided through the installation of Mountain Lion on to your freshly reformated hard drive

Hopefully this was helpful. All of the information I presented above is available in some form out on the internet, but I was suprised that I couldn’t find all of it in one place for those of us that have the need to completely erase and reinstall. Now it’s all in one place...

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