When using macOS, you certainly know if you’ve opened a Pages file or are editing an email. But, below the surface are files that macOS uses that you don’t see all the activity required for background processes, Time Machine backups, Spotlight indexing, and other system needs. When you want to eject a mounted volume, like a volume on an external SSD or hard drive, you can run afoul of macOS’s hidden needs.

You can try to eject a volume normally in these ways:

• Select it in the Finder and press Command-E
• Select it in the Finder and choose File > Eject
• Control-click/right-click the volume in the Finder and choose Eject

If you’re lucky, when you try to eject a volume, you will see a message with specific details, such as: The disk "My Photos" couldn't be ejected because "Adobe Photoshop 2024" is using it. Quitting Adobe Photoshop will let you eject “My Photos”, but if there’s something happening at a system level, you will see a message worded like this: The disk "My Photos" wasn’t ejected because one or more programs may be using it. You can try to eject the disk again or click Force Eject to eject it immediately. Cancel and Force Eject buttons are part of the dialog, and a progress spinner appears next to the phrase “Trying to eject.”. That does not help resolve the situation. Here’s what you can do to narrow down the problem:

Close Finder windows. As odd as it seems, sometimes merely having a window open in the Finder that shows the contents of the volume or a folder on it may make the Finder believe that a “program” is using that mounted item. Close the Finder window and try again.

Remove the volume from Spotlight. I’ve found that Spotlight can sometimes seize hold of a volume and mark it as unejectable even when indexing isn’t actively in process on that volume. You can check if that’s the problem by going to Apple menu  > System Settings... > Siri & Spotlight, scrolling down to the bottom of the view, and clicking Spotlight Privacy. Drag the volume into the window (or click + and navigate to it, then add it), and click Done. This immediately removes the volume from indexing. Try to eject now. If it works, that was the problem. However, if you want to search items on that volume, you will need to remove it from Spotlight Privacy later.

Check if Time Machine is backing up. If Time Machine is actively backing up the volume, you may be unable to eject it. Check the status of Time Machine either through its system menu or by going to Apple menu  > System Settings... > General > Time Machine. If it’s underway, you can choose Skip This Backup from the Time Machine system menu or click the “x” box next to the active session in the System Settings view and wait for Time Machine to wind down. You can exclude the volume from Time Machine by using the settings view: click Options, click the + (plus) sign at the bottom of Exclude from Backups, and add the volume while mounted. Click Done.

Shut down. For a physically connected external volume, choose Apple menu  > Shut Down. When your Mac is completely shut down, unplug the SSD or HDD. Press the power button on your Mac to start up. For a “logically” mounted volume, such as a disk image or networked volume, choose Apple menu  > Restart. On restart, the volume should be unmounted by default. If not, immediately select it and use an eject option listed above, which should now work.