MacPaw has released CleanMyDrive 2 (http://macpaw.com/cleanmydrive), a free (though it does offer in-app purchases) tool for reclaiming free space and clean junk off of external drives and flash drives. It requires OS X Yosemite or El Capitan.
CleanMyMac 2 has a completely redesigned user interface. Other new features include: file copying to any disk with drag-and-drop simplicity; detailed storage legend for every drive; mount and unmount notifications; automatic disk eject on system sleep initiation; a keyboard shortcut for mass disk eject; improved hidden service files cleanup; and automatic cleanup on every disk eject. You can also set custom disk icons from MacPaw’s crafted icon sets.