I mount my SD card by clicking on it in the sidebar of dolhin. When I want to unmount over the sidebar, Dolphin tells me that the target is still in use. Also it should work, when I'm in that SD card directory (but I am not) the command lsof tells me that Dolphin is still having access to it: dolphin 16701 cwd DIR 8,81 131072 2 /media/pietz/7000-8000/DCIM dolphin 16701 16702 QXcbEvent cwd DIR 8,81 131072 2 /media/pietz/7000-8000/DCIM dolphin 16701 16703 QDBusConn cwd DIR 8,81 131072 2 /media/pietz/7000-8000/DCIM dolphin 16701 16704 QThread cwd DIR 8,81 131072 2 /media/pietz/7000-8000/DCIM There is only one instance open. When I do "kill dolphin", I can unmount it over the sidebar with a fresh Dolphin instance.
I recently had a similar problem, although I unmounted from the tray icon and was in the directory of the external medium. After closing Dolphin, unmounting worked. Not sure whether the problems have the same root. However, it seemed to me as if image files contained on the medium were responsible. Looking at your posted output I see a DCIM folder, so likely images are also on your medium. Still this might just be a correlation.