Finder unable to move files. Makes copies only Since the update from Mavericks to Yosemite I have a very strange and annoying problem.

When I move a bunch of files to an external harddisk (drag &amp; drop) the last file of the group is not moved. 
It is only copied.
The original file remains at its source! I have to delete it manually afterwards. (Sometimes I forget to do so!)

I also found a workaround for this problem:
Move the files to the desktop first and from the desktop to the external drive.
This works most of the time.

Does anyone has the same problem?
Any ideas how to fix this?
If it's not a permission issue you can also try to reset Finder's preferences by deleting ~/Library/Preferences/com.apple.finder.plist

Restart your Mac to verify if the problem was resolved.