Reinstalling macOS
Fix your faulty system files by reinstalling the OS
Once you have backed up your data, you can eliminate any corrupted files by erasing the Mac’s hard drive and reinstalling macOS.
This is done in the macOS Recovery system. This is a separate area of your hard drive which contains tools to diagnose and fix problems such as the system running slowly or failing to start up.
Access the Recovery system by powering on your Mac and holding down ç+R. With luck you’ll see a welcome screen in various languages and an arrow button you can click to continue.
If for any reason Recovery fails to load, restart your Mac, start up with ç+å+R held instead. You’ll see a message stating ‘Starting Internet Recovery’. If you see the Choose Network option, select your Wi-Fi network and enter its password to download Recovery.
Once the Recovery system has loaded, double-click on the fourth option, Disk Utility. Disconnect your back-up drive before going further, so you don’t accidentally wipe it.
Inside Disk Utility
Erasing is a scary word, so it helps to think of resetting your Mac back to its factory settings. Once Disk Utility has loaded, you can select your hard drive in the left pane and run First Aid to try to fix damaged sectors. Note, though, that this process also runs when you start the Mac in Safe Mode, as you did in the previous section, so it may not make a difference.
It’s also possible to reinstall macOS ‘in place’ which replaces only system files and leaves your personal files untouched. However, this is time consuming and not guaranteed to fix the issues you’ve been experiencing. Ensure your Mac’s usual startup disk drive is selected on the left. This is usually named ‘Macintosh HD’. Next, click the Erase button in the toolbar.
Make sure that the format of the drive is ‘Mac OS Extended (Journaled)’, then click the Erase button, bottom right. You’ll be asked to confirm then the drive will be erased. This should only take a few seconds.
Quit Disk Utility and you’ll be returned to the macOS Utilities window. If you connect to the internet via Wi-Fi, look at the top right corner of the screen and click on the Wi-Fi icon. Check that there is a black tick next to your own Wi-Fi network. If not, click it and enter your password.
Next, click Continue to begin reinstalling macOS. Click Continue again on the setup page and then on the blue Continue button. macOS Recovery will check which system
If you have a recent Time Machine backup, you can restore your user account using Migration Assistant
your Mac is eligible to run. Click to agree to the terms and conditions. You’ll now be asked where you wish to install macOS. Choose the drive you erased moments ago, unless you want to install macOS on an external drive (see above), then click Install.
The Recovery system will download the version of macOS you had installed to your hard drive; the time taken will vary depending on your connection speed.
Setup Assistant
Once the reinstall has completed, the Mac will go through the Setup Assistant, just as it did the day you first bought it. The options here are fairly self-explanatory and you can change any of these after the process is complete. If you have a recent Time Machine backup, you can quickly restore your user account using Migration Assistant. This only copies across personal files, such as Documents and Pictures and Application settings, without affecting the system files themselves. Connect your Time Machine drive to the Mac before proceeding.
In the Transfer Information section of the Setup Assistant, the system will ask if you want to transfer information ‘From another Mac, Time Machine Backup or Startup Disk’. Select this and then click on your Time Machine drive when it appears, then click Continue at the bottom of the screen. If your backups were encrypted, the system will ask for their password. The next page asks which info to transfer. It’s best only to select your home folder – leave Applications, Other Files, and Computer & Network Settings unchecked. You can reinstall Applications from the Mac App Store or download them from the internet.
The Setup Assistant will begin ‘migrating’ (copying) data from your most recent Time Machine backup to the computer. Once it’s complete you’ll be able to log in with your previous password. If you use your Apple ID to log in and have since changed your password, it will be the same password you used at the time you ran the backup; after logging in it’ll update to your new one.
If you have erased your hard drive and reinstalled macOS, this should have eliminated any software issues. If the system is still slow and you chose to migrate your data from a previous backup, try repeating the above steps and set up an entirely new user account without any of your data. This will help you know for sure whether the problem is with your Mac’s hardware.
If you’re still experiencing issues, then it’s best to have your device examined at a Genius Bar or an Authorised Service Provider (locate.apple.com).