I found the answer :
- in the first default startup screen where you choose to open or create a project, uncheck the box "always ask me on startup"
- open a project, then in Studio go to preferences/startup/workspaces
- manage here your workspaces
- close and reopen OpenStudio : you now get to a new startup screen where you can choose your workspace.
Unfortunatly, there still is a bug
When I choose a different workspace in this screen, it says it has changed and have to restart, so I clic on "restart", but it then directly opens the Studio ... with the last project I used, in the other workspace.
The only workaround is to click on "manage connections" on startup screen and manually change the directory, a bit messy. Talend Can you fix that please ?