How to Stop Macro From Requiring File Open Prompt When Changing External WB Reference?

The following macro works okay with the exception of prompting me to open the file location over 500 times for each of the 500 formulas that I copy over from Column B to Column C. Also, after pasting the formulas the macro uses Find/Replace to update the file path from the previous month to the current month. Finally, the formulas reference a closed external workbook:

Sub Macro2()
'
LastRow = Cells(Rows.Count, "B").End(xlUp).Row

   ScreenUpdating = False
   
    Range("B3:B" & LastRow).Select
    Application.CutCopyMode = False
    Selection.Copy
    Range("C3:C" & LastRow).Select
    Selection.PasteSpecial Paste:=xlPasteFormulas, Operation:=xlNone, _
        SkipBlanks:=False, Transpose:=False

    Columns("C").Replace What:="September", Replacement:="October", SearchOrder:=xlByColumns

    ScreenUpdating = True
'    
End Sub

I thought that adding "ScreenUpdating = False" would suppress these open file prompts, but it doesn't seem to be having an effect. Is there a solution to suppress these prompts?