When I am using multiple workbooks, and have data in one book, and a sumif
function in the other I am receiving an error. When the data source is open
everything works fine, but should I close the external data source then re
open and update the workbook which has the SUMIF function then I receive a
#VALUE error.

=sumif([Workbook.xls]Sheet1'!$A$1:$A$3,=1,[Workbook.xls]Sheet1'!$B$1:B$3)

I would appreciate any suggestions on this matter.