I am trying to write a macro that will use one file to update another. I got
some test code to work the way I wanted, but I need to user to select the
target file, and that is causing trouble.

Here is what I have so far:
Option Explicit
Sub auto_open()
Dim FileName As Variant
Application.ScreenUpdating = False
FileName =Application.GetOpenFilename(MultiSelect:=False)
Workbooks.Open FileName
Workbooks(FileName).Worksheets("Sheet1").Unprotect Password:="password"
ThisWorkbook.Worksheets("Sheet1").Activate
ActiveSheet.Cells.Copy
Workbooks(FileName).Worksheets("Sheet1").Range("A1 ").PasteSpecial
Workbooks(FileName).Worksheets("Sheet1").Activate
ActiveSheet.Range("A1").Select
Workbooks(FileName).Worksheets("Sheet1").Protect Password:="password"
Application.CutCopyMode = False
Application.ScreenUpdating = True
ThisWorkbook.Close savechanges:=False
End Sub