Originally Posted by
James Kehoe1 What kind of data would I Iose?
It would restore the phone to factory settings. Meaning you'd lose all data. If you're worried about losing data conduct a backup first from settings.
However, to be perfectly honest, if you're worried about losing important data it means you installed the Insider Preview on the wrong phone. Insider Previews can and usually will go wrong. Sometimes in small ways, sometimes in big ways. Don't ever put unreleased pre-alpha builds on a phone you can't afford to lose.
Besides, a hard reset is recommended anyways after installing such a build.