Preventing unmount of external volume on MacBook Air

Hi Guys,

I have an external hard drive (WD My Book) that I am trying to run Disk Utility's First Aid to check it, but this fails because "First Aid could not unmount one of the other volumes in the volume's container".

When trying to unmount this volume I get the message "Failed to unmount volume because it is currently in use".

I am getting these errors even after rebooting the MacBook Air, with no software running except that which starts automatically. The only one that should be using the external volume is Backblaze, but that only takes a moment to run because I have not made any changes to the data on the drive, which is storing photos. The only way to Unmount the volume seems to be through a Force Unmount, which works, but then the problem comes back again when the volume is remounted. I am getting a bit worried about this and would like First Aid to be able to check the volume for errors. Is there any way to find out what is preventing the volume from being unmounted, what is "currently in use"?

In an old thread I did see mention that this can be caused by a file still showing as being open, but I did not it mentioned how to find the offending file, if this is the problem.

[Re-Titled by Moderator]