Difference between "diskutil unmount" and "diskutil unmountDisk"?
From man diskutil:

 unmount | umount [force] device
     Unmount a single volume.  Force will force-unmount the volume (less kind to any open
     files; see also umount (8)).

 unmountDisk | umountDisk [force] device
     Unmount an entire disk (all volumes).  Force will force-unmount the volumes (less kind
     to any open files; see also umount (8)).  You should specify a whole disk, but all
     volumes of the whole disk are attempted to be unmounted even if you specify a parti-
     tion.


So diskutil unmount just ejects a single volume/partition, diskutil unmountDisk the whole disk (including all volumes/partitions).