How to Hard Reset a Mac OS X Computer & Reinstall the Operating System

A fact of life is that computers slow down. Sometimes it’s due to wear and tear but it can also be something as simple as your hard drive full of files that are no longer needed. Or essential operating system files are accidentally deleted.

When this happens, it’s time to consider reinstalling the operating system. It’s a big pain in the neck because it’s not a short process, but in the case of macOS, it’s an easy one. However, you do need an Internet connection so don’t even think about doing this on the bus or anything.

This is something I’ve been meaning to do for a while, but procrastination is my friend. But today, for the sake of this article, I decided to finish it.

Step One – Backup All Necessary Files

This is always the first step before reinstalling the operating system. To delete all unnecessary files, back up the rest on cloud storage, USB stick or portable hard drive.

Illustration of a laptop backing up a folder to the cloud

Remember to also back up your iTunes library, iMovie database, and Photos database. They can be pulled into removable storage and then pulled back to the computer after the process is over.

If you use Time Machine, this backup process is easy.

Step Two – Turn off FileVault

Enabling FileVault prevents you from reformatting and reinstalling the hard drive. So let’s go in System Preferences–> Security & Privacy and turn it off. This process can take up to 30 minutes so be patient. Go make coffee or something.

Step 3 – Have you encrypted the boot disk?

For security reasons, you should encrypt your startup disk in the first place. The slight downside to this is that if you forget the encryption password, you can never unlock it again and can never reinstall macOS.

Believe me, I am speaking from very bitter past experience here.

MacBook Air operating system installation screen

Assuming you know your password, restart your computer and simultaneously hold down CMD + R. This will then show you the above padlock screen (I had to take a picture because I didn’t). can take screenshots at this stage).

Enter your password and the screen will then change to show you this. Again, I had to take pictures with my iPhone so sorry for the not-so-perfect quality.

If you don’t know your password then you’re really out of luck because even Apple won’t unlock it for you.

macOS Utility Options

Step 4 – Erase the contents of the hard drive

Erase button specified in Disk Utility window

As you can see from the menu above, there is an option called “Disk Utility”. Select it and then select the disk where the operating system is installed. In my case there was only one disk but if you are dual booting you will have more than one.

Now click “Erase” and a small box will pop up asking you to enter the desired name of the newly formatted drive as well as the file format type (APFS). I recommend leaving them as they are.

Remove the Macintosh HD pop-up confirmation window

The deletion actually takes a few seconds (in my experience anyway). When done, the “Used” portion of the disk will be very small (in my case 20KB). At this point, everything on your computer is gone.

Close the Disk Utility window and you’ll be returned to the Utilities screen.

Step Five – Choose your preferred reinstall option

Now there are actually two options in the Utilities window that you can choose from.

The first is a Time Machine backup. If you’re in the habit of regularly backing up with Time Machine and one day you accidentally delete all of your system files, you can simply restore your computer to a Time Machine backup from the previous day. This is similar to performing a System Restore on a Windows PC.

But I don’t use Time Machine (I back up manually). So for me and others like me, the only other option is to select the “Reinstall macOS” option. So go ahead and click that and click “Continue” when it prompts you.

Step Six – Pretend to read the User Agreement

You will now be asked to read the user agreement. Do what everyone else does and pretend you read it and click “Agree”. Don’t worry, Apple will never know.

macOS Mojave

Now choose a disk to install the operating system on. In my case there was only one disk. Select it and continue.

macOS Mojave screen where you select the disc you want to install it on

The reinstall will now begin.

The computer will restart several times during this process, and it may take up to an hour or more to complete. The nice thing is that it does everything on its own from now on, so you can get up and running in the meantime. You’re not stuck staring at a screen as your life slips by.

Step 7 – Reset Everything Again

Once the system has been reinstalled, you’ll have to start the tedious process of getting everything back the way it was. This will include:

  • Moving on Firewall.
  • Switch on FileVault.
  • Re-encrypt the boot disk.
  • Reinstall your application.
  • Bring the necessary files back to your computer from your backup.
  • Add a screen lock PIN.

Basically you have to go through System Preferences and check everything one by one. The computer is now back to factory settings, so any tweaks and customizations you made earlier will be gone.

There is a great tutorial called Hardening macOS [note link not working] offers you a huge list (40+) of security precautions you should take when installing fresh macOS. I highly recommend you refer to it and do as much as you can. Some of that may sound like overkill, but you can never be too careful.

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