As someone recently asked me how to  unmount an external disk:

a) by forcing it to unmount: sudo unmount <harddisk>
e.g. sudo unmount /Volumes/hdd_xyz
…which is really dangerous because you force your disk controller to throw out your drive even it is working for you… (risk of dataloss):/

b) [Better solution]
find your real enemy: sudo lsof | grep /Volumes/xyz and try to stop, eliminate those working, open files from your disk drive <xyz>