If 'External' means another open workbook then try the below
'Change the workbook name as required. Only saved workbook will have the
extension.
Sub Macro()
Dim lngRow As Long, wsDest As Worksheet, rngTemp As Range
Set wsDest = Workbooks("workbookname.xls").Sheets("Sheet2") 'Destination sheet
Set rngTemp = Sheets("Sheet1").Range("A1:A3")
lngRow = wsDest.Cells(Rows.Count, "A").End(xlUp).Row + 1
rngTemp.Copy wsDest.Range("A" & lngRow)
End Sub
If you mean closed workbook; try the below..
Sub Macro()
Dim lngRow As Long, rngTemp As Range
Dim wbBook As Workbook, wsDest As Worksheet
Set rngTemp = ActiveSheet.Range("A1:A3")
Set wbBook = Workbooks.Open("<fullpath>\<filename.ext>")
Set wsDest = wbBook.Sheets("Sheet1") 'Destination sheet
lngRow = wsDest.Cells(Rows.Count, "A").End(xlUp).Row + 1
rngTemp.Copy wsDest.Range("A" & lngRow)
wbBook.Close True
End Sub
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