The reason why they were left in the unfinished dir is most likely that you didn't have permissions to write to the mount location after the volume was unmounted.
In general this also happens when destination dir becomes full. So I'd say the core should save the files to be copied somewhere (like Queue.xml) and retry the copy later when destination/more space becomes available. Instead of just leaving files to unfinished dir and discarding the directory structure.
The reason why they were left in the unfinished dir is most likely that you didn't have permissions to write to the mount location after the volume was unmounted.
In general this also happens when destination dir becomes full. So I'd say the core should save the files to be copied somewhere (like Queue.xml) and retry the copy later when destination/more space becomes available. Instead of just leaving files to unfinished dir and discarding the directory structure.
--RZ