Drag and Drop in File Manager Does not work?

I just installed Aurora yesterday, and so far, most of the common desktop issue can found some explanations online. But one behavior confuses me.

If I attached an external drive to my laptop, and I try to drag one file in the external drive to a folder inside the home directory, say the Download directory, Dolphin would show the progress and the copied file would show up in the destination folder.

However, if I dismount the external drive, then the copied file would also disappear in the destination directory.

What’s behind this behavior?