Continuing the discussion from Removed project from list, With no backup.:
This is a HUGE bug (*) where the Hub will simply, silently, irrevocably remove ALL projects from the Hub’s project list if that project’s path is unavailable at the time the Hub launches.
This means you will irrevocably lose any Hub project in the following situations:
- Renaming or moving the project folder
- Renaming any path leading up to the project folder
- Not having the removable drive (with projects on it) mounted at the time of launching the Hub
… and then also restarting the Hub / Computer.
This is super annoying, disastrous behaviour on behalf of the Hub. 
Why it does not simply show a “project currently unavailable” notification instead is beyond me.
For the time being, be sure to make frequent backups of this file if you want to be able to restore lost projects:
C:\Users\{USERNAME}\AppData\Roaming\UnityHub\projects-v1.json
(*) In my opinion this is a fatal bug that loses a user’s data! Though Unity responded saying it’s “by design”. I asked them to reconsider. I wanted to post this here so perhaps there’s a greater chance this gets reconsidered.