When Google Drive is the destination, by default we only keep user sharing for those users who both belong on the source domain and also exist at the destination.
Therefore, for a scenario where users do not exist on the destination, it is necessary to add the following Advanced Option: MigrateExternalUserPermissions=1
With this flag set, the following will occur:
- If the user is known in the source domain but doesn't exist at the destination yet we know the email address of the destination user, we migrate it using "external sharing".
- Note: This requires an email notification to the destination email address of the user during migration.
- If we don't have the email address of the destination user during migration (this is possible during a Google Drive to Google Drive migration), then we migrate the sharing based on the user ID.
It's important to warn customers who want to enable this option that, in the case of external sharing, we will send an email notification to external users. This is a Google API requirement.