The disk couldn’t be ejected because the Finder is using it

Hello mattia225,

Thank you for using Apple Support Communities! I understand that you're encountering a message that your external HD can't be ejected because Finder is using it, however you've quit all open applications in your Activity Monitor. You've come to the right place for help.

The next time that you encounter this message, I'd like to have you relaunch Finder, then try to eject the external HD again:

How to force an app to quit on your Mac - Apple Support

• "You can also force the Finder to quit, if it stops responding. Select Finder in the Force Quit window, then click Relaunch."

If there's no change in behavior, run First Aid using Disk Utility on the external HD next:

Repair a storage device in Disk Utility on Mac - Apple Support

Keep us posted on the results.

Cheers!

There is the concept of an in-use bit. If you have a file opened from that external drive, or you have actually told Finder to navigate to a folder location on it, the operating system will not eject it because it is "in-use." For instance, I have a folder mounted from my DiskStation NAS, and I clicked a folder on it from the Finder. The following command in the Terminal, confirms that Finder is still accessing that mounted volume:

lsof /Volumes/video
COMMAND PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
Finder  428 viking   29r   DIR  52,29      264    2 /Volumes/video

If I click on the Finder Desktop, the in-use bit returns to 0 and the mounted volume is ejectable.