I tried an experiment: I copied two files from my internal drive to an external and then tried, unsuccessfully, to unmount the external. I quit and relaunched Finder with the same lack of success. Logging out/in also was unsuccessful. It seems that only a restart allowed me to eject the external.

This is not a big deal, since restarting takes about 15 seconds with my machine. I don't know if this information is useful, but there it is.

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365