When an external hard drive suddenly stops mounting on your Mac, or your internal system drive starts throwing unexpected read/write errors, the hardware is not necessarily broken. Often, the issue is simply file system corruption caused by a sudden power loss or an improperly ejected cable. Before concluding that your data is gone forever, you must attempt to repair the drive using macOS Disk Utility.
Disk Utility is a native system application capable of diagnosing and repairing directory structures and file system errors (a process formerly known as “Repair Disk Permissions”). In this guide, you will learn how to use the First Aid feature to rescue corrupted drives.
Launching Disk Utility
If your Mac boots normally, but an external USB drive is failing, you can run Disk Utility from your main desktop.
- Open Finder.
- Navigate to Applications > Utilities.
- Double-click Disk Utility.
What if the Mac Won’t Boot?
If the corrupted drive is your actual internal macOS system drive, and your Mac is trapped on the Apple logo screen, you must launch Disk Utility from macOS Recovery Mode.
- On an Apple Silicon Mac (M1/M2/M3): Shut down the Mac. Press and hold the power button until you see “Loading startup options”. Click Options, then click Continue.
- On an Intel Mac: Shut down the Mac. Press the power button, then immediately press and hold Command + R until you see the Apple logo.
Once the Recovery menu appears, select Disk Utility and click Continue.
Running the First Aid Diagnostic
Whether you launched it from the desktop or Recovery Mode, the interface is identical.
- Look at the left-hand sidebar. This lists all physical drives and logical volumes currently connected to the Mac.
- Crucial Step: Click the View button in the top-left corner of the window and select Show All Devices. This ensures you can see the root hardware (the actual physical disk) rather than just the storage container.
- Select the corrupted drive from the sidebar. (It is usually highlighted in grey if it is failing to mount).
- Click the First Aid button in the top toolbar (the stethoscope icon).
- A warning dialog will appear stating that First Aid will check the volume for errors and repair the volume if necessary. Click Run.
Understanding the First Aid Process
The repair process can take anywhere from a few seconds to several hours, depending on the severity of the corruption and the size of the drive. Do not interrupt this process or unplug the drive.
You can click the Show Details triangle to watch the terminal output as macOS verifies the catalog file, the multi-linked files, and the volume bitmap.
Interpreting the Results
- Green Checkmark (Success): The volume appears to be OK. The file system was repaired. The drive should now mount normally in Finder.
- Red Exclamation Mark (Failure): Disk Utility cannot repair the disk. This usually means the corruption is too severe, or the drive has suffered physical mechanical failure. If you receive this error, you must immediately copy as much data off the drive as possible (if it mounts at all) and replace the hardware.
By relying on macOS Disk Utility’s First Aid feature, you can often recover critical data from external drives and resolve boot failures without resorting to expensive third-party data recovery software.