Preventing unmount of external volume on MacBook Air

Thanks very much - that's the magic command that I needed. The result of this is that I need to ask an additional question. The source of the problem looks to be Spotlight. As noted in my original message, I am not actively changing the content on my external volume, so Spotlight indexes on it should be up-to-date. I think Spotlight is having a problem. My MBA has been attached to the external volume all night, I used the Kill command to terminate the Spotlight process with its PID returned from the Lsof command, and tried to use Disk Utility's First Aid on the volume, which resulted in the same error messages as before. Running the Lsof command shows that Spotlight is running again on the volume. There seems to have been some evolution of the processes running, based on changing PID values, but they seem to have settled at those shown in the screenshot below. Is this an indication of Spotlight misbehaving and, if so, what can I do about it?

Thanks for the feedback. My feeling is that something is borked with Spotlight. I know that my wife will just unplug the laptop from connected drives without Ejecting/Unmounting them first, so I don’t know if doing that has the potential to corrupt such that I get this issue with Spotlight. I am assuming here that, if there are no changes to be indexed, Spotlight should finish running very quickly. Since I did not used to have this issue Unmounting the external volume, something has changed and gone wrong.

Thanks very much. I had shutdown the laptop completely last night, which I don't do very often. This evening, when I started it back up to try the advice, I did find that Disk Utility was now able to run First Aid on the problematic external volume, which happily did not show any errors. I have "reset" the Spotlight index on the drive, so it is now re-indexing it. I am not sure why shutting down the laptop appeared to have the desired effect that rebooting did not.

Right now, I think all is good, so the advice given is appreciated.

Just to confirm, Disk Utility was refusing to Unmount the volume, which was certainly part of the problem. This is what was generating the error message shown in the screenshot in the original post. The lsof command did highlight that Spotlight was the process that was using the drive, although why this would prevent Disk Utility to Unmount the volume is unclear. I could not see any processes in the list returned that looked like they were anything not associated with Spotlight.

Anyway, all seems to be working as expected this evening.