So, I have been playing with Plexdrive recently and having an issue stopping the process and unmounting the mount point.
When I executed
$ fusermount -u /path/to/plexdrive
The command failed with an error
fusermount: failed to unmount /path/to/plexdrive: Device or resource busy
This is basically happen when the mount directory left mounted due to a crash. In my case, I forcefully killing the Plexdrive process.
The solution is pretty easy. What you have to do is just execute
$ sudo umount -l /path/to/plexdrive
and you are good to go.