Disk Utility's First Aid is designed to detect and repair file system issues on a disk, such as repairing the drive's data structures, fixing external hard drives not mount on Mac. However, when the damage is too severe, the repair process may fail, and macOS will return an error message.
Common messages and failures include:
- The first aid process has failed to unmount the volume for repair
- Problems were found with the partition map, which might prevent booting
- First Aid found corruption that needs to be repaired
- File system verify or repair failed (-69845)→ severe corruption
- Invalid B-tree node size→ broken file index
- Couldn't mount disk (-69842)→ system cannot access files
When this happens, the key is not just fixing the error, but understanding your situation first and choosing the right action to avoid data loss.
Check Your Situation First
Before fixing anything, quickly check your current situation in the table below.
| Situation | What it means | What to do |
| Drive opens normally | Disk still readable | Back up files immediately |
| Drive is read-only | Early corruption | Copy important data now |
| Drive won't mount | File system likely broken | Go to data recovery |
| Disk is slow / disconnecting | Possible hardware issue | Stop repairs, save data |
Warning: Do not write new data to the disk. This may overwrite recoverable files.
Methods to Fix Disk Utility First Aid Failed Errors
If First Aid fails, you can still try several repair methods depending on the cause. These methods progress from basic system repair to deeper repair of the corrupted external hard drive on Mac, so it's best to follow them in order.
Method 1: Run First Aid Again
- Open Disk Utility.
- Click View → Show All Devices.
- Select the external disk.
- Click First Aid → Run.
If First Aid fails with errors like 'could not unmount the volume', it means the disk is still in use and cannot be repaired in normal mode. Running First Aid from macOS Recovery avoids this issue.
- Restart your Mac.
- Immediately press and hold:
Command + R (Intel Mac)
Power button → Options (Apple Silicon Mac) - When macOS Recovery loads, select Disk Utility.
- Choose your external drive.
- Click First Aid → Run.
If First Aid completes → the disk is repaired. If it still fails → the disk is likely severely corrupted.
Method 2: Run FSCK
FSCK (File System Consistency Check) is the underlying tool macOS uses to check and repair disk errors. Disk Utility's First Aid is based on it, but running FSCK directly can perform a deeper repair when First Aid fails.
Note: Single User Mode is not available on newer Apple Silicon Macs. Alternatively, go to macOS Recovery Mode, select Utilities > Terminal, and then enter this command: /sbin/fsck_hfs -fy /dev/disk3s2
- Open Disk Utility.
- Select your external drive, then note:
Device name (e.g., disk3s2)
File system format (e.g., HFS+, exFAT)
- Restart your Mac, then immediately hold Command + S to enter Single User Mode.
- When you see the command line, type the appropriate command:
/sbin/fsck_hfs -fy /dev/disk3s2 Before running the command, update it based on your disk:
Replace disk3s2 with your disk's identifier (found in Disk Utility, e.g., disk2s1)
Replace fsck_hfs with the correct tool for your disk format:
Use fsck_apfs for APFS
Use fsck_exfat for exFAT
Use fsck_msdos for FAT32
Use the command that matches your disk format + your disk name, not the example shown above.
- Wait for the process to complete.
If you see 'File system was modified' or 'The volume appears to be OK '. The disk has been repaired successfully. If no success message appears, stop further repair attempts and move to data recovery.
What to Do If Disk Utility First Aid Still Fails
If none of the repair methods work, it usually means the file system is too damaged to fix.
At this stage, continuing to repair the disk may lead to further data loss. The priority should shift from fixing the disk to protecting your data.
Recover Data from the External Hard Drive
When the file system is too damaged, macOS cannot display files even if they still exist on the disk. In this situation, data recovery software can scan the disk, recovering data from the corrupted external hard drive directly.
iBoysoft Data Recovery for Mac is a tool that supports USB flash drive data recovery and SD card data recovery, and is suitable for:
- Unmountable or unreadable drives
- Corrupted or formatted disks
- APFS encrypted volumes
- Download, install, and open iBoysoft Data Recovery for Mac.
- Select the external hard drive and click Search for Lost Data to start scanning.
- After scanning, click Preview to check files.
- Select the needed files and click Recover.
- Save recovered files to a different disk (not the original drive).
Reformat the External Hard Drive
After recovering your data, reformatting is usually necessary when the file system is too damaged to repair. This is typically indicated when tools like Disk Utility First Aid and FSCK both fail or repeatedly return errors (such as “File system verify or repair failed”).
In these cases, the disk's core structures cannot be reliably fixed, so reformatting rebuilds them from scratch and removes the corruption.
However, this process will erase all data on the disk, so make sure your files are recovered or backed up first.
- Open Disk Utility
- Select the external drive
- Click Erase
- Enter a name and choose a format:
APFS (Mac only)
exFAT (Mac and Windows) - Click Erase and wait for completion
How to Prevent This in the Future
- Always eject drives safely
- Avoid sudden power loss
- Keep regular backups
- Check disk health periodically
FAQs
- QCan a failed external hard drive be repaired?
-
A
It depends on what caused the failed external hard drive. If the external hard drive failed due to software problems such as file system corruption or soft bad sectors, it is possible to repair it and get it back to a workable state. However, if the failed external hard drive is physically damaged, you need to replace the hardware to see if it can be fixed.
- QCan you recover a failed external hard drive?
-
A
You can recover files from a failed external hard drive with the right data recovery tools or the help of professional data recovery services. The success chance of recovery varies according to how damaged your external hard disk is.