Disk Utility Error in macOS: Causes and Fixes

What Does a Disk Utility Error in macOS Mean?

Disk Utility is a built-in macOS application for managing disks and partitions. Errors in it typically manifest as:

• "Could not check/unmount disk" — the utility cannot obtain exclusive access to the disk.
• "Operation failed" — the First Aid process was interrupted by a critical error.
• Disk doesn't appear in the list — the system does not recognize the physical drive.
• "Volume is damaged" or "Needs repair" — the file system is in an inconsistent state.

An error usually appears when attempting to run First Aid, unmount, or create a partition. Symptom: a pop-up window with an error code (e.g., -69877) or a simple text message.

Common Causes

1. Active disk usage. The disk is busy with system processes (Spotlight, Time Machine), applications, or even Finder.
2. File system corruption. Improper disk ejection, power failure, or a hardware drive error.
3. FileVault conflict. If the disk is encrypted and FileVault is active, Disk Utility may not be able to access it without a password.
4. Incorrect permissions. Your user account does not have read/write rights to the root partitions.
5. Hardware issues. Faulty cable (USB/Thunderbolt), port, or the drive itself.
6. System limitations. The GPT partition structure is violated, or the disk header is damaged.
7. Conflict with SIP (System Integrity Protection). In rare cases, security policies block low-level operations.

Solutions

Method 1: Standard First Aid from Recovery Mode

This is the most effective method because the utility runs on a minimal system without background macOS processes.

1. Restart your Mac into Recovery Mode:
   • For Apple Silicon Mac: shut down, hold the power button until startup options appear, select "Options" → "Continue in Recovery Mode."
   • For Intel Mac: turn on and immediately hold ⌘ + R.
2. From the menu, select Disk Utility.
3. In the left column, select the physical disk (the top item with the model name, e.g., "APPLE SSD AP0256J"), not the partition underneath it.
4. Click the First Aid button and confirm.
5. Wait for completion. If First Aid reports success, reboot into the normal system.

Method 2: Force Check via Terminal

If the graphical interface won't launch or crashes, use the command line.

1. Boot into Recovery Mode (as in Method 1).
2. From the menu, select Terminal.
3. Find the disk identifier:

   diskutil list
   

   The output shows all disks. Find yours (e.g., /dev/disk2). Note the partition (e.g., disk2s1).
4. For APFS disks (modern Macs):

   sudo fsck_apfs -y /dev/disk2s1
   

   The -y flag automatically answers "yes" to repair prompts.
5. For Mac OS Extended (HFS+) disks:

   sudo fsck_hfs -fy /dev/disk2s1
   

6. For exFAT (external drives):

   sudo fsck_exfat -fy /dev/disk2s1
   

7. After completion, type reboot to restart.

⚠️ Important: Always specify the partition identifier (s1, s2, etc.), not just the disk (disk2). Using the wrong identifier can damage other partitions.

Method 3: Manually Unmount the Disk

If you get a "Could not unmount disk" error, try forcibly unmounting it via Terminal before running First Aid.

1. Boot into the normal system or Recovery Mode.
2. Determine the disk's mount point via diskutil list (e.g., /Volumes/MyDisk).
3. Force unmount:

   sudo diskutil unmountDisk force /dev/disk2
   

   (replace disk2 with your disk identifier).
4. Now try running First Aid in Disk Utility again.

Method 4: Temporarily Disable System Integrity Protection (SIP)

Only use this if other methods failed and you understand the risks. SIP protects system files but can interfere with deep recovery operations.

1. Boot into Recovery Mode.
2. Open Terminal.
3. Disable SIP:

   csrutil disable
   

4. Reboot into the normal system and try First Aid or the Terminal check again.
5. After successful recovery, must return to Recovery Mode and re-enable SIP:

   csrutil enable
   

Method 5: Hardware Diagnostics and SMC/NVRAM Reset

If the issue occurs on multiple disks or the disk remains undetected after all actions:

1. Check the cable and port. Try a different cable (especially for USB-C) and a different port.
2. Reset SMC (for Intel Mac) or NVRAM/PRAM:
   • SMC: shut down Mac, hold Shift + Control + Option + Power for 10 seconds.
   • NVRAM: turn on Mac and immediately hold Option + Command + P + R for about 20 seconds.
3. Run Apple Diagnostics (hold D at startup) to check the hardware.

Prevention

• Always eject disks safely. Use "Eject" in Finder or diskutil eject /dev/diskX in Terminal.
• Don't interrupt write operations. Don't shut down your Mac or disconnect the disk during copying, backup, or verification.
• Regularly back up your data. Use Time Machine or third-party solutions.
• Keep macOS updated. Apple regularly releases fixes for storage subsystems.
• Monitor SSD health. For SSDs, use utilities like smartmontools (Terminal: sudo smartctl -a /dev/diskX) to check SMART attributes.
• Avoid unnecessary fragmentation. Fragmentation isn't critical for SSDs, but for HDDs keep at least 15% free space.

What to Do If the Disk Doesn't Recover?

If no method helps and the disk continues to report errors, it's most likely a hardware failure:

1. Create a disk image for potential data recovery attempts:

   sudo dd if=/dev/disk2 of=/path/to/backup.dmg bs=4m conv=noerror,sync
   

   (replace disk2 and the save path). The process will be very slow and may stop on errors.
2. Consult data recovery specialists. The less you interact with the damaged disk, the higher the chance of recovery.
3. Replace the drive. For MacBook/Air, this may require visiting an Apple Service Center.

Summary: Disk Utility errors almost always relate to the file system state or the disk's hardware. Start with First Aid from Recovery Mode, then move to Terminal utilities. Backups are your primary defense against disk corruption.