I started the initial backup on my iMac, expecting it to take a looooong time, as the selected data came to over 1.7TB, which is quite a lot. In the interim period, I may have confused it a bit by removing around 300GB of duplicate and no longer needed data, taking it down to a bit under 1.5TB.
At one point, the backup stalled, with messages suggesting I was offline (I wasn’t), but as my MacBook was showing the same thing, I suspected this was a temporary communication error, and sure enough by the following morning, it was happily chugging away.
From then, I checked periodically to see how it was doing, and earlier this week, I saw that it had indeed finished, as you can see:
It now automagically keeps backing up new and changed files, which is a Good Thing.