On June 2, 2017 I had an innocent message of from Win10 Pro that updates were pending. That Win10 had been dormant in my Mac Pro for about 6 months, and the update was the Anniversary Edition (AE) update. The AE update behaved like a cross between a full system reinstallation and a virus.
It sought out my one of my HFS partitions, obliterating the drive, and touched two NTFS data-only drives (the primary and backup, of course) trashing the directories in things younger than 6 months. That's why I remember the date; it was truly traumatic. It took me four days to run variety of data recovery utilities to get everything back.
The takeaway from that experience is that you should install Win10 with only the target drive connected. It's not a matter of choosing a wrong target, it's the unpredictable behavior of the install routines that can wreak havoc. (Is anything else ever wreaked?)