Polar posted this link for sync problems under Windows 10 which is what @
whojammyflip posted above.
https://support.polar.com/en/v650-sy...-to-windows-10
The 1.5.4 firmware (April 25, 2018) has made a big difference in reliability of syncing for me.
https://support.polar.com/en/updates...irmware-update
-Tim-