It doesn`t happen if you defrag it in safe mode which is what you should be doing. In "Normal" mode the HDD is constantly being written to and defrag restarts all the time taking hours longer.
Surface Pro (2017) vs Surface Pro 4
Where HTML5 is headed next
MacBook Pro v Surface Pro 5
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.