Reinstall El Capitan or Later Without Erasing Drive
Please be sure you back up, if possible.
- Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Utility Menu appears.
- Select Disk Utility and click on the Continue button.
- Select the indented (usually, Macintosh HD) volume entry from the side list.
- Click on the First Aid icon in the toolbar. Wait until the Done button activates, then click on it.
- Quit Disk Utility and return to the Utility Menu.
- Select Reinstall OS X and click on the Continue button.
If the above doesn't work:
Install El Capitan or Later from Scratch
If possible, back up your files.
- Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:
- Select Disk Utility from the Utility Menu and click on the Continue button.
- When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
- Click on the Erase icon in Disk Utility's main window. A panel will drop down.
- Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
- Click on the Apply button, then wait for the Done button to activate and click on it.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.