It appears the automation error had something to do with visibility of the window in which the external workbook was displayed. When I opened the workbook and then made the window invisible, the automation error disappeared. Thing is, I wanted the window invisible all along, but left if visible because I thought it would facilitate debugging efforts (debugging an earlier problem).

Anyway, the more I think about it, the more I remember that program flow depends upon visibility. I'm no VBA expert, but hiding a form causes execution to return to the next visible form or workbook, right? So maybe by leaving the workbook visible, I was inadvertently screwing up the flow of program control through the system. And maybe making the window invisible corrected the control flow, thereby eliminating the automation error.

Or maybe it was the pentagram that I drew around my computer to ward off the evil automation error spirits.

Your post concerning the need to destroying any and all links to the workbook before closing it made me go back and check to make sure all my references to ranges in the external workbooks were terminated (set to nothing) before I closed the workbook. Not sure if that helped, it did not eliminate the automation error, but it seems a likely thing to do and I suspect in so doing I've avoided future problems in some way.

In any case, I thank you very much for your help.