What does “couldn’t unmount disk” mean on a Mac?

The “Couldn't Unmount Disk on Mac” message (don’t confuse it with the Disk Not Ejected Properly error) occurs when your Mac is blocking you from unmounting your disk because it’s corrupted, a process is using it at the time, or it is being modified. For instance, this can happen while attempting to erase or partition your boot disk in Disk Utility.

How to fix couldn't unmount disk on Mac

There are several ways to fix the “Couldn't Unmount Disk” error. Use Disk Utility in Recovery Mode—this is the safest solution. You can also unmount the disk via Terminal, though it’s a tricky process. Another option is to force quit current processes via Activity Monitor. Use third-party disk management tools.

1. Use Disk Utility in Recovery Mode

Fixing the “Couldn't Unmount Disk” error requires modifying the internal drive from a different volume. You can achieve this by accessing macOS Disk Utility via an external hard drive or Recovery Mode. Note that the process of booting into Recovery Mode differs depending on your Mac model.

This is how we restarted macOS into Recovery Mode to unmount the disk using Disk Utility:

On Apple Silicon:

1. Turn off your Mac and hold down the Power button, releasing it when the loading startup options appear on the screen.
2. Click on Options > Continue.
3. Enter your username and password.

On an Intel Mac:

1. Switch off your Mac and leave it alone for a few seconds.
2. Turn it back on while holding down the Command R keys, releasing them when the Apple logo appears on the screen.
3. The macOS Utilities window should now appear—choose Disk Utility to unmount the drive.

2. Unmount disk via Terminal

Mac’s Terminal utility is another option to unmount your disk. You can also use it in Recovery Mode to unmount the disk you’re currently booted on. Just ensure you use the right disk identifier to avoid losing your data.

These are the steps we took to manually unmount a disk using Mac’s Terminal app:

1. Choose Terminal in Recovery Mode, Spotlight search, or select Finder > Applications > Utilities > Terminal.
2. Enter the command diskutil list and hit Return.
3. Make a note of the identifier for the disk you want to unmount, enter the command sudo diskutil unmountDisk /dev/[identifier] (replace the square brackets with the name of the drive), and press Return again. You may have to enter your administrator’s password.

3. Force quit processes via Activity Monitor

If active processes are preventing your Mac from unmounting the disk, force quitting them in Activity Monitor could do the trick.

Here’s how we identified and closed active processes in Activity Monitor:

1. Open Finder > Applications > Utilities > Activity Monitor.
2. Ensure the CPU tab is selected at the top of the window and review the list of processes at the top of the list (those are the ones consuming the most resources).
3. Right-click on an active process, hit the X icon at the top of the screen, and select the Force Quit option on the dialog box.

4. Use third-party disk management tools

When all else fails, consider using a third-party disk management tool to unmount your disk. It does what Mac can’t do, which is forcibly unmounting the disk due to ongoing system processes. Several software options exist in the market. One of these is EjectBar, which can unmount multiple disks in one click. Another option is Jettison, which automatically ejects external disks before your Mac goes to sleep.

Here’s where they come in handy:

• When Disk Utility fails to unmount your disk
• When your disk has been stuck "in use" state, even when it isn’t in use
• When hidden system files are blocking your external drive from ejecting

We can’t tell you which one to use, as different users have unique preferences. But we recommend searching for viable solutions in the App Store and weighing your options.

Safely unmount the disk on your Mac

A disk refusing to unmount on your Mac can be problematic, especially when you’ve never experienced this before. Fortunately, this guide lists several fixes you can try to successfully and safely unmount your disk. For instance, you can unmount your disk using Recovery Mode. If you’re comfortable using Terminal commands, Mac’s Terminal app is another option. Activity Monitor is another simple tool for force-quitting active processes. Finally, you can forcibly unmount your disk using a third-party app.

Remember, the simplest method to safely unmount your Mac’s disk is to use MacKeeper’s Premium Services. It’s a perfect solution that connects you to tech experts who patiently help fix Mac system issues like the “Couldn't Unmount Disk” error you’ve received in real time, no waiting on the phone line for assistance.

At MacKeeper, we’re passionate about helping you fix common Mac errors, so you can enjoy using your Mac. Check out our full blog for more helpful guides, like how to fix the "Select the disk where to install macOS" blank error.

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