I've noticed, quite a few times that thunar crashes or becomes unresponsive ..... especially when it comes to mounting/unmounting external devices.
This especially happens when mounting large USB3 devices and wanting to see what\s on there....leaving thunar totally unresponsive upto even unable to close it, requiring "xkill" to get rid of it.
While thunar is in it's unresponsive state, copying files to and from the device using the command line or "mc" works fine ..... as well as unmounting with "sudo"... so IMO it's thunar itself that's at fault.
After xkilling thunar, it will not restart unless the whole system is rebooted.
This is not what you might call good behavior, let alone stable.