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If you're trying to repair a malfunctioning external hard drive or flash drive, plug it into one of your computer's USB ports.  Skip this step if you're attempting to fix a built-in hard drive. You may need a USB 3 to USB-C adapter if your Mac doesn't have traditional USB ports. It's a menu item at the top of your Mac's screen. A drop-down menu will appear. If you don't see Go at the top of the screen, click the Finder app icon in your Mac's dock (or click the desktop) to force it to appear. You'll find this option near the bottom of the drop-down menu. Double-click the Disk Utility app icon, which resembles a grey hard drive with a stethoscope on it. In the upper-left corner of the window, click the hard drive that you want to fix. It's an image of a stethoscope at the top of the Disk Utility window. Doing so will prompt Disk Utility to begin scanning (and repairing) your selected hard drive's bad sectors. Once the Disk Utility finishes repairing your drive, you should see a pop-up explaining what was repaired. If you don't see any repairs listed, your hard drive doesn't have any bad sectors to repair. For each time that a repair (or series of repairs) is listed, run Disk Utility again to scan for further issues. Once Disk Utility no longer reports repairs upon completing the scan, your Mac's hard drive is fixed.
Connect the drive to your computer if necessary. Click Go. Click Utilities. Open Disk Utility. Select a hard drive. Click the First Aid tab. Click Run when prompted. Wait for the repairs to complete. Run Disk Utility again.