When attempting to test File Connect (connecting to Google Drive) as another user, I'm getting the error message
Can't access external source
External data source is unavailable because of an error.
I have confirmed that the user has the Files Connect Cloud permission on the profile and the profile also has access to the External Data Source. Testing with my own system administrator user works fine, it's just when logged in as another user that I am getting this.
What I found out with this is that Files Connect Google Drive doesn't appear to work when you are using the 'Log in as' feature to log into Salesforce as another user.
In order to test this functionality as another user, you must log in directly as that user - using 'Log in as' through the Setup will cause this error.