Do you have System Restore turned on? This uses some hard disk space in case you need to revert your system to an earlier time. It regularly creates System Restore points - not sure if it is every day though.
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.