You cannot take a SharePoint external list offline in SharePoint Workspace 2010 if the list resides on SharePoint Server 2010 Standard Edition or on SharePoint Foundation

Consider the following scenario:

• You are running Microsoft Office 2010 and Microsoft SharePoint Server 2010, Standard Edition or SharePoint Foundation.
• You create a SharePoint external list on the SharePoint server.
• You try to sync the external list to Microsoft SharePoint Workspace 2010.

In this scenario, the SharePoint external list is not taken offline as expected. You may also receive the following error message:

Additionally, Event Viewer displays an error that includes the following information:

This issue occurs when you take an external list offline from a server that is running the Standard edition of SharePoint Server 2010, or from a server that is running SharePoint Foundation. External lists use Business Connectivity Services (BCS) applications. Therefore, you must have the Enterprise client license available on the server in order to take an external list offline. Because these SharePoint servers cannot provide authorization for the client computer to run the application, your attempt to take the list offline fails. 

To resolve this issue, create the external list on a server that is running the Enterprise edition of SharePoint Server 2010. Then, take the external list offline from that server.

External lists use BCS applications. Therefore, there are more rigorous licensing and technical requirements for taking an external list offline than for taking other kinds of lists offline. For more information, see the "Using external lists in a SharePoint workspace" topic in SharePoint Workspace Help.