Sharing setting for an external object

Instead of using web services or the Data Loader to put the data into Salesforce, we can get up-to-date data directly from the third-party system by using an external data source. By using an external data source, and the external object it creates, we can access information located outside Salesforce as if it were stored inside Salesforce. Below are the important points in terms of sharing of external Object:

1. The external data source is read-only, so we cannot use it to update data on a third-party system
2. Apex sharing reason is not available for external objects.
3. Record-level security (sharing) not supported
4. Field accessibility (field access settings) not supported

In short, sharing is not supported for external objects and it is only read-only so you can control the sharing from the profile. If you want to show external object data to a specific profile, you can specify that profile and hide for other profiles.