Excel 2007 creates a circular reference

I have an error (or don’t understand the workings of the new version of
excel). I have a workbook G:\2009\Monthly Reporting Working Files\P05\Trial
Balance\YTD Financials.xlsx. This workbook is linked to the period 04 version
workbook (G:\2009\Monthly Reporting Working Files\P04\Trial Balance\YTD
Financials.xlsx). Note it is has the same file name but is held in a
different location.

It links to cell D47 of period 4 and then adds current month. The formula
works ok but when I save the document and reopen it the links get broken and
the cells become a circular reference. I can’t understand why.

I have the drive set up in my trusted locations, I am using the correct file
extension in my formula and my security settings are set on prompt for
workbook links.

Does anyone know how to find out how to ensure the formla is not changed?
Note: this was not a problem in Excel 2003