Suppose you want to programmatically access data in a SharePoint external list (e.g. data from SQL Server). You may think that you can just use SharePoint’s ListData.svc. Think again…
I did some experimenting with an external list connected to SQL Server, with the following results:
|ListData.svc (REST)||Not supported||Not supported|
|Web services (SOAP)||Supported (GetListItems)||Not supported|
|Client side object model||Supported||Supported|
In short: if you want CRUD access to an external list, your SharePoint API choice is reduced to the client side object model.
I hope this is useful to others working with external lists, as there is little information about this topic on the internet. Let me know if you have different findings.