I'm getting this error - this workbook contains links to one or more external sources that could be unsafe

As far as I know, this message is a Security Warning that Automatic update of links has been disabled.
To suppress the message and to automatically update the links when you open a workbook, follow these steps:   

1. Click Microsoft Office File, and then click Options.
2. Click Advanced.
3. Under General, click to clear the Ask to update automatic links check box.

https://support.microsoft.com/en-us/kb/826921 (It also applies to Excel 2013)