The below works - but is there a better way? It seems that I had to add the
..select to make it work regardless on sheet was previously active - then I
forced another worksheet to be active.

Sub AddNewWeek()
Dim LastRow As Long
shtGraphData.Select
With Sheets("Graph_Data")
If Range("Z6").Value Range("Z5").Value Then
LastRow = .Cells(.Rows.Count, "K").End(xlUp).Row
.Cells(LastRow + 1, "K").Value = Range("Z6").Value
.Cells(LastRow, "L").Resize(1, 6).Copy .Cells(LastRow + 1, "L")
End If
End With
shtInput.Select
End Sub