Filling References to Another Workbook

When you use Excel’s Paste tool to create a reference to a cell in another Excel workbook (not another worksheet in the same workbook), Excel creates the reference in this manner:

='[Sales Master.xls]Sheet1'!$A$5

The presence of the dollar signs means that this is an absolute reference to the cell. Because of this, you cannot use any of Excel’s automatic fill options, such as Fill Right (Ctrl+R) or Fill Down (Ctrl+D) or AutoFill, and get the results you expect. Instead, every cell in the filled cells will reference the exact same cell in the external workbook.
The solution to the problem is to make a quick modification to the referencing formula before you do the fill. If you remove the dollar signs (both of them), then the formula is now relative, and filling will work the way you expect.