SUMMARY
I connect external hard drive to my laptop with Neon distribution. In Dolphin appears proper entry I click into it and in /media/neon  get mounted resource calling (in my case) "My Passport". I open couple media files and close it after some time. Sometime I open in also in Midnight Commander. After I finish I got out from mounted media directory and in Dolphin I change director,  or even close it. Now when I try to "Safety remove" my external storage in Dolphin I get error message saying that "One or more files on this device are open within an application.".
Trying to rid off this message I close all my applications and run Dolphin again with Home directory. Now again try to "Safety remove", but it doesn't help. Error still persists

Please notice that error message is not so clear, because says about some files. User don't know which file or directory is opened.
The worse is I cannot unmount resource. Quick investigation brings me the answer that resource is blocked by library calling "file.so". I suppose Dolphin use it and it is not unload from some unknown reason. Just after I kill the process, I'm able to "Safety remove" (unmount) resource.

Investigation relies on calling command lsof, what in my case looks like this:
$ lsof | grep media
file.so    2193                             neon    5r      REG               8,49 328056931     130881 /media/neon/My Passport/Piotrek/video/my_movie.mp4

STEPS TO REPRODUCE
1. Via USB connect external storage
2. Open it in Dolphin clicking into entry showed in left panel "Places"
3. Open couple media files, for example movies, and close them
4. Change directory in Dolphin and restart it
5. Try to "Safety remove" connected storage in Dolphin

OBSERVED RESULT
Error message saying that "One or more files on this device are open within an application." caused by library: "file.so".

EXPECTED RESULT
User should be able to "Safety remove" (unmount) connected storage when any file or directory in opened in any application he used. Here Dolphin or mc.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.21.80
KDE Frameworks Version: 5.80.0 (or current in Neon)
Qt Version: 5.15.2

ADDITIONAL INFORMATION