Open Workbook Using Name In Cell & Copy Data

I have a VBA-coded macro in an Excel workbook that is to open another existing workbook -- a .xls file, not .csv -- copy data from it, paste that into the original workbook where the macro resides, then close the source workbook.

If I use the debugger to step through the macro -- putting a breakpoint at its first executable line, run the macro, then F8 step through it, it works fine. (With delays at the open, close statements.) Free running, it doesn't. I suspect it's a timing issue, that the macro runs full tilt even though the new workbook isn't yet open -- but I don't know how to remedy that.

Here is the situation and the simplified code (the actual macro does a lot more, of course):

Main workbook: Two sheets, "Parameters" that has the fully-qualified filename for the desired source .xls in A1; and "Data", into which I want to paste data from that source.

Source workbook: Single sheet, "Sheet1"

Macro code. For simplicity, it is trying to copy/paste all of the source worksheet's content:

Sub Go()
'
' Go Macro
' Test of getting data from another spreadsheet
'
' Keyboard Shortcut: Ctrl+Shift+G
'
Dim SourceFile As String
Dim HomeBook As String
Dim OtherBook As String
Sheets("Parameters").Select
SourceFile = Range("A1").Value
HomeBook = ActiveWorkbook.Name

right after the open, hoping that might cause a dwell for the open to complete.

When I step through it, the Locals view has all the parameters, getting set properly. If I try to simply run it, it stalls at the opened source workbook, the Locals view is empty, no data has been transferred into the initiating workbook.

If I place the breakpoint after the open, it is not reached. (I have the VBA open.) Hitting F8 then yields a VB error window, "Compile Error" "Expected: To"