Excel VBA, getting range from an inactive sheet

I encountered a problem like this myself: I was trying to search through a separate worksheet to see if the color of a cell matched the color of a cell in a list and return a string value: if you are using .Cells(row, column), you only need this: Sheets("sheetname").Cells(row, column) to reference that range of cells.

I was looping through a block of 500 cells and it works surprisingly quickly for me.

I have not tried this with .Copy, but I would assume it would work the same way.

Your issue is that the because the Cell references inside the Range 's are unqualified, they refer to a default sheet, which may not be the sheet you intend. For standard modules, the ThisWorkbook module, custom classes and user form modules, the defeault is the ActiveSheet. For Worksheet code behind modules, it's that worksheet.

For modules other than worksheet code behind modules, your code is actually saying

Sheets("Temp").Range(ActiveSheet.Cells(1), ActiveSheet.Cells(1).End(xlDown)).Copy
Sheets("Overview").Range("C40").PasteSpecial

For worksheet code behind modules, your code is actually saying

Sheets("Temp").Range(Me.Cells(1), Me.Cells(1).End(xlDown)).Copy
Sheets("Overview").Range("C40").PasteSpecial

In either case, the solution is the same: fully qualify the range references with the required workbook:

Dim sh1 As Worksheet
Dim sh2 As Worksheet

Set sh1 = ActiveWorkbook.Sheets("Temp")
Set sh2 = ActiveWorkbook.Sheets("Overview")

With sh1
    .Range(.Cells(1,1), .Cells(1,1).End(xlDown)).Copy
End With
sh2.Range("C40").PasteSpecial

Note: When using .End(xlDown) there is a danger that this will result in a range extending further than you expect. It's better to use .End(xlUp) if your sheet layout allows. If not, check the referenced cell and the cell below for Empty first.