Copying a range of data into another Master workbook below existing data

Maybe:

Sub Grizz()
Workbooks("Weekly.xls").Sheets("Sheet1").Range("C13:D" & Range("C" & Rows.count).End(3)(1).Row).Copy Workbooks("Master.xls").Sheets("Sheet1").Range("A" & Range("B" & Rows.count).End(3)(1).Row)
End Sub