Connecting External Lists to SharePoint Workspace: Tips and Warnings
Applies to: SharePoint Server 2010
To be able to see the Sync to Computer button on the Microsoft SharePoint Server Ribbon and to take an external list offline to Microsoft SharePoint Workspace, Offline Sync must be enabled for the external content type. This can be easily done using Microsoft SharePoint Designer 2010.
Also note that if the external list is not upsized, that is, if it uses the default browser forms, and you take the list offline to SharePoint Workspace, SharePoint Workspace will auto-generate the InfoPath forms for you on the client. However, note that on the server, the list will continue to use the default browser forms unless you specifically upsize the list.