Took some 3 hours to download the map, by which time the OS had timed out and unmounted my 1030. The next morning I unplugged it, plugged it back in, and it apparently installed correctly.

However, after my next ride, something strange happened during sync. On the subsequent ride, I didn't have any HR readings (other sensors were fine). When I got home the unit uploaded the ride over WiFi, and by the time I'd pulled out an old unit to check the HRM, it registered heart rate again. Seems to have worked since then without problems.

Could be calculating a checksum, in my instance, or some other silent check during sync. Might it be an ISP problem (dynamic host control protocol changing IP address mid-stream?) or map file too big for your computers?