Rsync mishap .. OS hdd is now full
There is nothing inherently magically going on within /Volumes. So if you use rsync or something else to copy data into that folder, it will just do as told. If there is no folder with the target name already it will just create a new one on your main drive. 

So

sudo rm -rf /Volumes/Hired-Hand


should remove the files/folders created by rsync. Just make sure you get the name right (and maybe eject the HiredHand drive first).