Windows 10 Pro x64. System has 2 disks:
Disk1: "C:" - Small SSD with system, and user accounts and documents (some folders syncing with resilio).
Disk2: "D:" - Larger disk with a large synced folder. Say "D:\Data"
"Default folder location:" was located at "C:\Users\<username>\SyncFolder" but was empty and NOT USED to sync anything.
System was running unattended, for several days.
When I checked today the system was choking for lack of disk space. It turned out that Resilio Sync had substituted for "D:\Data" a folder "C:\Users\<username>\SyncFolder\Data"
As a result, disk C: was completely full and several important processes had crashed or hanged for lack of disk space.
Apart from "D:\Data", two other subfolders of "C:\Users\<username>" were syncing, they were substituted for subfolders of "C:\Users\<username>\SyncFolder".
When I say "substituted", I mean that the original folders had been left intact but new subfolders of "C:\Users\<username>\SyncFolder" were created with the same base name and were now syncing, in lieu of the original ones.
PS: This system had been automatically upgraded to Windows 10 Pro "version 1709". I had decided to roll it back to "version 1703", and postpone feature upgrades for a year.