I was surprised to find this but this is a gem for anyone struggling with Excel Services errors. Microsoft made a flow chart of what Excel Services does during a data refresh. This helped me debug errors that made no sense at one point.
Quote from the link above:
“Use this flowchart to better understand how Excel Services in Microsoft SharePoint Server 2010 refreshes data or to troubleshoot data connection errors from Excel Services.
The Excel Services Data Refresh Flowchart shows the sequence of actions performed by Excel Services when it refreshes data in a data-connected Microsoft Excel workbook that is being rendered by Excel Services. This includes sequences for the three available authentication options (Windows, Secure Store Service, and None), plus sequences for embedded connections and connections that use Office Data Connection files. The flowchart also includes the connection errors that can occur at each step, and it suggests configuration steps to avoid or correct the errors.”
Hope this helps someone.