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 macOS Recovery, 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 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 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 the Recovery system.
Once Recovery has loaded, double-click on the fourth option, Disk Utility. Disconnect your backup drive and others before going further, so you don’t accidentally wipe any of them.
Inside Disk Utility
Erasing is a scary word, so it helps to think of this process as 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 the damage. 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 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. Once you confirm, the drive will be