If you leave this value set to the default (i.e. Yes), then you’ll be greeted with the following message for each Excel Web Part on a TFS dashboard page:

This workbook contains one or more queries that refresh external data. A malicious user can design a query to access confidential information and distribute it to other users or perform other harmful actions.

If you trust the source of this workbook, click Yes to enable queries to external data in this workbook. If you are not sure, click No so that changes are not applied to your workbook.

Do you want to enable queries to external data in this workbook?

To avoid warnings before refreshing external data in Excel Services:

  1. On the SharePoint Central Administration home page, in the Application Management section, click Manage service applications.
  2. On the Service Applications page, click Excel Services Application.
  3. On the Manage Excel Services Application page, click Trusted File Locations.
  4. On the Excel Services Application Trusted File Locations page, point to the trusted file location that you want to edit (e.g. http://), click the arrow that appears, and then click Edit.
  5. On the Excel Services Application Edit Trusted File Location page, in the External Data section, clear the Refresh warning enabled checkbox, and then click OK.
Manage Excel Services trusted file locations