The solution is that every separate WordPress installation will need to use its own unique Google API project.
Note: this doesn’t mean a unique Google account – it means a separate project within your Google account’s API console. (See the Google Drive setup guide to refresh your memory on this, if it’s unfamiliar).
This isn’t officially documented by Google anywhere – as far as we know (the documentation is voluminous, and distributed across many places). The only thing you would know from the official guides is that each WordPress install needs a unique client ID. However, several users have found that when they use the same project for several sites, then Google Drive randomly sends back access errors. This probably means that there’s a glitch some inside Google’s own code. But, whatever the reason, it’s not something that can be addressed from this end – you will need to use separate projects in order to work around it.
Posted in: Google Drive