You need to run it as administrator. By default it flushes internal drives. If you want to use the switches to flush external drives you can run it from an administrator command prompt (or make a shortcut that includes the switches).
I used this tutorial to run as administrator without the UAC pop-up - Elevated Program Shortcut - Create for Standard User - Windows 7 Help Forums
It always flashes quickly - don't worry about that. Just check it is working OK with admin command prompt as below..
The target path for the shortcut to run as administrator is below (you will need to change the bits in red to your computer name and where you have saved the sync.exe object)
It works on Ten Preview as well. hth
C:\Windows\System32\runas.exe /user:Adam-X201\Administrator /savecred "C:\Users\adam\OneDrive\Windows Software\sync.exe"