When your Mac starts experiencing severe data corruption—files randomly disappearing, applications crashing upon launch, or the entire operating system refusing to boot—the hardware itself might not be failing. Often, the file system structure has simply become scrambled. Before you panic and book a costly repair appointment, you should try using macOS’s built-in repair tool: Disk Utility First Aid.
Disk Utility is a powerful system application that manages hard drives, SSDs, and external volumes. The “First Aid” feature scans the file system for directory errors, permissions issues, and corrupted data structures, and attempts to repair them automatically. In this guide, you will learn how to run First Aid safely.
Running First Aid on External Drives or Non-System Volumes
If you are trying to repair an external USB drive or a secondary partition that does not contain the macOS operating system, the process is straightforward and can be done while you are using your Mac.
- Press Cmd + Space to open Spotlight, type Disk Utility, and press Enter.
- In the left sidebar, locate the drive you want to repair.
- Tip: Click the View button in the top-left toolbar and select Show All Devices. This ensures you see both the physical hardware disk and the logical volumes nested beneath it.
- Select the lowest volume in the hierarchy for that specific drive first.
- Click the First Aid button in the top toolbar (it looks like a stethoscope).
- A warning will appear stating that the volume will be temporarily locked. Click Run.
- If the process completes successfully, a green checkmark will appear. If you selected “Show All Devices”, you should now work your way up the hierarchy, selecting the parent disk and running First Aid again.
Running First Aid on Your Startup Disk (macOS Recovery)
If your main internal hard drive (usually named “Macintosh HD”) is corrupted, you cannot run a full repair while macOS is currently running off it. It is like trying to change a car’s tyre while driving. You must boot your Mac into macOS Recovery mode.
Step 1: Enter macOS Recovery
The method depends on your Mac’s processor.
- Apple Silicon (M1/M2/M3) Macs: Shut down the Mac completely. Press and hold the power button until the screen says “Loading startup options.” Click Options, then click Continue.
- Intel Macs: Shut down the Mac completely. Press the power button, then immediately press and hold Cmd + R until you see the Apple logo.
Step 2: Run First Aid
Once the Recovery interface loads, you will see a window titled “macOS Utilities”.
- Select Disk Utility and click Continue.
- Click View > Show All Devices.
- In the sidebar, locate your internal drive (usually labeled “Macintosh HD – Data” or just “Macintosh HD”).
- Crucial Step: Start at the very bottom of the hierarchy for your internal drive. Select the lowest nested volume.
- Click First Aid and then Run.
- Once it completes, select the volume directly above it in the sidebar and run First Aid again.
- Continue working your way up until you have run First Aid on the physical disk at the very top of the list (e.g., “APPLE SSD…”).
Understanding the Results
When First Aid finishes, you can click Show Details to read the log.
- Success: If it reports “The volume appears to be OK” or “File system was modified” followed by a success message, the corruption has been fixed. Restart your Mac normally.
- Failure: If it reports “First Aid found corruption that needs to be repaired” but then fails, or “The underlying task reported failure on exit,” the file system is too damaged for an automated fix. At this point, you must back up your data immediately (if possible), erase the drive completely, and reinstall macOS.
By understanding how to boot into Recovery and run Disk Utility properly, you can often save a failing Mac from software-level destruction in just a few minutes.