It is a fact that computers slow down. Sometimes it is due to wear, but it can also be something as simple as filling your hard drive with files that are no longer needed. Or essential operating system files that are accidentally deleted.

When that happens, it’s time to consider reinstalling the operating system. It’s a huge pain in the neck as it’s not a short process, but in the case of macOS it’s an easy process. You need an internet connection, so don’t think about doing it on the bus or anything.

Mac laptop on a marble worktop

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

Step One: Back up all essential files

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

Illustration of a laptop backing up a folder in the cloud

Remember to backup your iTunes library, iMovie database and Photos database as well. These can be dragged to portable memory and then dragged back to the computer at a later time when this process is finished.

If you use Time Machine, this backup process is very simple.

Step Two: Disable FileVault

Having FileVault turned on prevents you from reformatting and reinstalling your hard drive. Then enter System Preferences–> Security and Privacy and turn it off. This can take up to 30 minutes, so be patient. Go get yourself a coffee or something.

Step Three: Have you encrypted your startup disk?

For security reasons, you should have encrypted your startup disk from the very beginning. The slight downside to this is that if you forget the encryption password, you will never be able to unlock it again and you will never be able to reinstall macOS.

Trust me, I’m speaking from very bitter past experience here.

MacBook Air operating system installation screen

Assuming you know your password, restart your computer and, at the same time, hold down the CMD + R keys. This will then show you the lock screen above (which I had to photograph as I can’t take screenshots at this stage).

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

If you don’t know your password, you are seriously out of luck as not even Apple will unlock it for you.

MacOS utility options

Step Four: Erase the contents of the hard drive

Erase button indicated in the Disk Utility window

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

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

Clears the Macintosh HD popup confirmation window

Erasing literally takes seconds (in my experience anyway). When finished, the “Used” part of the disk should be tiny (in my case, 20KB). At this point, everything on your computer is gone.

Close the Disk Utility window and you will be redirected to the Utilities screen.

Step Five: Choose your preferred reinstall option

There are now actually two options in the Utilities window that you can choose from.

The first is Time Machine backup. If you’re in the habit of backing up with Time Machine regularly and one day you accidentally delete a whole bunch of system files, you can just restore your computer to a Time Machine backup, for example, the day before. This would be the equivalent of performing a System Restore on a Windows PC.

But I don’t use Time Machine (I do the backup manually). So for me and others like me, the only other option is to choose the “Reinstall macOS” option. So go ahead and click on that and click “Continue” when prompted.

Step Six: Pretend to read the user agreement

Now you will be asked to read the terms of use. Do what everyone else is doing and pretend you’ve read it and click “Agree”. Don’t worry, Apple will never know.

macOS Mojave

Now choose a disk on which to install the operating system. In my case, there is only one disc. Choose it and continue.

MacOS Mojave screen where you select the disk you want to install it on

The reinstallation process will now begin.

The computer will restart several times during the process and can take up to an hour or more to complete. The great thing is that from now on he does everything himself, so you can go and do something else in the meantime. You’re not stuck staring at the screen watching your life slip away.

Step Seven: Reset everything again

Once the system has been reinstalled, you will have to begin the tedious process of getting things back to the way they were. This will include:

  • Turn on the firewall.
  • Activating FileVault.
  • Re-encrypt the startup disk.
  • Reinstall your apps.
  • Bring essential files back to your computer from backups.
  • Adding a screen lock PIN code.

Basically you have to go through System Preferences and check everything one by one. The computer is now back to factory settings, so all previously made changes and customizations will be lost.

There is a great guide called Hardening macOS [note link not working] which gives you a huge list (more than 40) of security precautions you should take with a fresh install of macOS. I highly recommend that you refer to it and make as many as possible. Some may seem excessive, but you can never be too careful.

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