MacBooks are highly prized for their smooth operation and high security level, but just like any other man-made machine, they’re not immune to errors. One issue that is majorly encountered by users is the notorious “Couldn’t Unmount Disk” error. The case is likely to occur when you try to delete, partition, or fix a disk with Disk Utility, and you see that your system won’t allow you to do so as the disk is either locked, or in use, or is damaged.
For the average user, this problem can be more than frustrating, it can strike at the most inopportune time when you’re attempting to reinstall macOS, make a bootable drive, or prepare a disk for a backup
In this article, we will go in-depth into why the “Couldn’t Unmount Disk” error happens in the first place.
Why “Couldn’t Unmount Disk” Error Appear on Mac?
To fix this issue, you need to dig slightly deeper as to what makes it occur. Unmount on macOS suggests detaching a disk or volume from the file system so that it can be erased, formatted, or repaired securely. It also serves several other important purposes like safe removal of external media, maintenance of system and troubleshooting, or switching between file systems or OS, meanwhile ensuring your data’s privacy & security. If the system won’t unmount, it could be a result of something prohibiting it within your system.
Let’s examine the key reasons:
1. System Disk in Use
If you are working on the startup disk of your Mac, you cannot unmount it from the macOS. The system reads and writes continuously on that disk for regular functioning, and thus when you attempt to erase or repartition it within Disk Utility, macOS prevents it. This is the most frequent cause of why individuals encounter the error.
2. Background Processes or Apps Blocking the Disk
For some other times the issue isn’t with the system itself but background services. Like,
- Time Machine could be running a backup.
- Spotlight could be accessing the disk.
- A third-party software for instance, antivirus or disk management could be scanning or locking files.
Even when you think that the disk is inactive, one of the above reasons might be holding the unmount.
3. File System Corruption or Partition Map Problems
Logical corruption or damaged partition maps on disks might result in unmount failures. For instance, if the APFS container structure is damaged or the HFS+ catalog file is corrupted, macOS might not be able to unmount it properly. That is a sign of more severe problems with the drive.
4. External Drive Connection Problems
For external disks, loose cables, malfunctioning USB hubs, or power problems can also stop safe unmounting. The OS receives conflicting messages and fails to unmount the disk to prevent possible data corruption.
5. Locked or Encrypted Volumes
In case you are dealing with an encrypted APFS volume and it has failed to be unlocked successfully, or if the keychain is holding it wrongly, the system may block the unmount request.
6. Severe Hardware Faults
In some instances, the drive itself might be failing physically. If macOS is unable to communicate with the disk controller, it will repeatedly eject with the unmount error.
How to Resolve the “Couldn’t Unmount Disk” Issue on Mac?
Now that you understand the potential causes, let’s go through the solutions in depth. Each of the following methods addresses a different causative factor.
1. Restart Your Mac and Retry
It seems basic, but restarting will get rid of processes that could be holding the disk. If it is a background service problem, this will solve the issue right away. Always attempt this before proceeding with more complex steps.
2. Boot macOS Recovery Mode
If the issue is with your startup disk, you will be compelled to operate outside your primary operating environment.
Shut down your Mac.
Press and hold the power button (Touch ID button on laptops) until you see “Loading startup options.”
When the options screen appears, click Options → Continue.
Enter your admin password if prompted, and you’ll be in macOS Recovery.
Once in Recovery Mode, launch Disk Utility.
Choose the disk and attempt to unmount.
In Recovery Mode, however, the system is not actually operating from your startup disk, which means you are more likely to succeed.
3. Use Disk Utility’s First Aid Tool
If corruption is suspected, administer First Aid. Here’s how:
- Under Disk Utility, choose the volume below the disk.
- Click on First Aid and let it scan for file system errors.
- Do the same for the container (if it’s APFS) and then device level.
- Reconstruction is done layer by layer, so every logical structure is tackled.
4. Unmount Using Terminal by Force
In stubborn situations where Disk Utility is being obstinate, Terminal might prove more useful.
But use it only if Disk Utility refuses to eject a disk.
- Run:
- Open Terminal from Applications → Utilities.
diskutil list
This shows all connected disks and their identifiers (e.g., /dev/disk2).
- To unmount the whole disk, run:
diskutil unmountDisk force /dev/diskX
Replace diskX with your actual disk number.
- If you only want to unmount a single volume, use:
diskutil unmount force /dev/diskXsY
Only use force if normal unmount fails. If any app is writing to the disk, forcing can corrupt data. Always double-check that nothing important is running before using it.
Be careful as compelling an unmount when data is actively being written can lead to corruption. Always make sure no critical operations are in progress.
5. Delete or Format the Disk
If the disk is still not available and you don’t need the information, you can still erase it. This though wipes everything off of the drive.
- Choose the disk in Disk Utility.
- Click Erase.
- Select the preferred file system.
- Click Continue.
Note: Do not delete the disk if there are significant files on it. Before deleting, ensure that you have created a backup of the data.
6. Inspect Hardware and Connection
For external drives. At times, you might have to remove the drive forcefully. Just make sure no read-write action is in the process to avoid data corruption. Even if “nothing is being written,” the OS may still be caching data. Better you first shut down the Mac before unplugging it.
7. Try Mac Data Recovery Tools as a Last Resort
If the above fails, chances are that the disk is badly corrupted or has hardware issues. Now, if you have lost your crucial data, you don’t have to give up hope. Attempt a professional-level recovery software such as Stellar Data Recovery for Mac.
Where Stellar Data Recovery for Mac Fits In
Though macOS utilities do their job well in terms of making speedy repairs, they have their limitations. In case your disk fails to unmount due to corruption, partition problems, or boot problems, native utilities might not suffice. That is where Stellar Data Recovery for Mac steps in:
Recover from Unmountable Drives: Even if Disk Utility won’t unmount or mount a drive, Stellar will be able to scan the drive directly at the device level.
Works on Non-Booting Macs: The application can be launched in recovery mode, which enables you to recover files when your Mac won’t boot up normally.
Advanced Scan Modes: Rapid scan for minor problems, or in-depth scan to recover lost data from severely damaged drives.
Create a Disk Image: In case your drive is physically failing, the software allows you to make a sector-by-sector copy and recover from the image in a secure way rather than stressing the failing hardware.
Supports Multiple File Systems: No matter what format the file is – your disk could be APFS, HFS+, FAT32, exFAT, or NTFS formatted, Stellar has got you covered.
Opting for Stellar before taking risky measures or seeking professional help, you can recover your important files, following which, you can proceed with repairs confidently.
Avoiding the Situation in the Future
Although certain disk errors are unavoidable, proper procedures can greatly minimize the risk of experiencing unmount issues.
Eject Disks Properly:
Always eject with the “Eject” option before unmounting an external drive.
You can eject the drive using any of the below mentioned ways:
Finder Sidebar: In Finder, look for your external drive under Locations. Click the small Eject (⏏️) icon next to it.
Desktop Icon: If the drive is visible on your desktop, drag it to the Trash. The Trash icon will change to an Eject symbol when you hover.
Right-Click Menu: Right-click the drive in Finder or on the desktop and choose Eject.
Disk Utility: Open Disk Utility, select the drive, then click the Eject button in the toolbar.
Keep Regular Backups:
iCloud, third-party backup software, or Time Machine guarantee that even if your disk crashes, your data is secure.
Practice First Aid Regularly:
Running Disk Utility’s First Aid on a regular basis assists in finding and repairing minor problems before they become enormous.
Don’t Overfill Drives:
Having less than 10% free space is unsafe. A disk that is almost full is more likely to corrupt.
Monitor Disk Health:
Use SMART monitoring software to verify your drive health. Replace the disk if you notice early failure warnings.
Key Takeaways
The “Couldn’t Unmount Disk” error might seem complicated to many, but it can be quickly fixed once you know the root cause and the perfect solution for it. From restarts to Recovery Mode maneuvers and Terminal commands, you have quite a few ways to correct it for starters.
However, if you find out that the disk is corrupted or your data is vulnerable, macOS’s built-in utilities may fall short. If data recovery is necessary, Stellar Data Recovery for Mac can be a valuable tool. By understanding the proper combination of troubleshooting and maintenance, you can lower downtime, keep your files safe, and maintain your Mac like it’s new.
