How can I suppress Open File prompt when copying over formula referencing closed workbooks

Thanks, Paul, for your response, but I already have it on manual calculate. Still, whenever the formula in each cell tries to pull the value from an external workbook, the file prompt opens even though I already have the correct file path in the formula. If this only happened once or twice it wouldn't be big deal, but I copy these formulas over for each month, and their must be at least 100 instances each month.