Actually, the field test Hubs (which @GaryFunk has) do not store data because they were not built with external flash. The production Hubs, however, do have external flash.
So, both the end device (AIR & SKY) and the the Hub will store data when necessary in a first-in, first-out buffer. The AIR & SKY will store several hours of data if the Hub is without power or out of range. And the Hub will store several weeks of data if the Internet connection is down. Once the connection is restored, the data will backfill.
- AIR will store about 2.5 hours of data (if it can’t reach the Hub)
- SKY will store about 1.5 hours of data (if it can’t reach the Hub)
- With one AIR & one SKY, Hub will store about 8 days of data (if it can’t reach the Internet)