Rarely, you may need to reinstall macOS Mojave system software, usually for troubleshooting purposes. Reinstalling MacOS Mojave as discussed here will aim to only reinstall the macOS Mojave system software itself, it will not erase the drive, nor will it remove any user data or user files, making it ideal for troubleshooting.
This walkthrough will show you the easiest way to reinstall macOS Mojave from a Mac that already has macOS Mojave on it by using Recovery Mode.
Note that reinstalling macOS Mojave with this approach is not the same thing as performing a clean install. A clean install of Mojave erases literally everything on the computer, including personal data, by using a bootable Mojave installer drive, while reinstalling aims to simply reinstall only the macOS operating system itself.
Be sure to backup your Mac first, it’s easy to back up with Time Machine. While reinstalling macOS Mojave system software only aims to only replace the system software portion of a computer, it’s possible that something could go wrong resulting in data loss. Therefore you absolutely must backup your entire Mac and all personal data before going any further.
How to Reinstall MacOS Mojave
Reinstalling macOS Mojave (10.14) is relatively simple:
- Backup the Mac before going any further, do not skip making a full backup
- Restart the Mac, then immediately hold down the COMMAND + R keys together immediately to boot into macOS Recovery Mode (alternatively, you can also hold down OPTION during boot and select Recovery from the boot menu)
- At the “macOS Utilities” screen, select “Reinstall macOS”
- Choose “Continue”, and then agree to the terms when asked
- Now at the drive selection screen, select “Macintosh HD” (or whichever drive you want to reinstall macOS Mojave onto) and choose “Continue” to begin the reinstallation process for macOS Mojave
- Reinstallation of macOS Mojave will begin and the Mac screen will turn black, showing an Apple logo with a progress bar that indicates roughly how far along the reinstallation process is, let the Mac run through this entire process undisturbed
* If the Mac has Filevault enabled with full disk encryption you will need to choose “Unlock” and enter the Filevault password before you can reinstall macOS Mojave onto the computer.
When installation has completed, the Mac will boot up as usual with freshly reinstalled macOS Mojave system software. You can then login with your normal user account as usual, complete with all of your personal files, applications, and other data.
After booting back up again it’s a good idea to run Software Update in macOS Mojave to install any available system software updates on the Mac.
If for some reason you are missing user accounts or personal data, then something probably went wrong in the process and you would probably want to restore from the Time Machine backup you had created prior to beginning this entire process. While that should not happen, it’s always theoretically possible for something to go wrong with any technical endeavor involving system software, or perhaps you added in extra steps on your own to format the drive (which is not necessary for simply reinstalling macOS, nor recommended unless you are aiming to erase everything and perform a clean install).
And that’s how you can reinstall macOS Mojave system software at any time. This should not be a necessary process for the vast majority of Mac users, but it can be a very helpful troubleshooting method if you find something has gone wrong in macOS Mojave either after a software update failure, or perhaps an inappropriate attempt at deleting tmp files and /private/var/ system folders or some other critical system component, a fairly unlikely situation but it’s always possible.