Device > Garmin Connect Account > [ ] "Erase data on this device after successfully syncing"
I see it now, it depends on which device is selected, it's not there if I select Vivofit or Virb, but it's there when I select Edge 810, 510. Mine was unchecked BTW.
I have been testing the 3.2.21 update to see if the "Erase data on this device after successfully syncing" box is being enabled automatically and I cannot reproduce that behavior. Since noticing this post and the other thread I installed 3.2.19 paired a 310XT and a 910XT. Created several one minute test activities on both which synced successfully, then updated Express to 3.2.21. The Erase data from device box remained unchecked on each device. Though I cannot say why the box would be enabled without direct input from the user, I can say its is not due specifically to the 3.2.21 update.