- ERROR RESOLVED -

The "Unable to read file" error was being generated by a combination of the
workbook being protected, a certain pivot table property and the SaveCopyAs
method.  I still don't understand how the SaveCopyAs method invokes this
situation whereas my pivot tables properties have not changed and the
workbook has always been protected without any issues.  Nevertheless it does
and here's how I resolved the error.  

~ right click pivot table and choose "Table Options"
~ UNcheck the box under the "Data Options" section labeled "Save Data with
Table Layout", click OK
~ repeat these steps for all pivot tables in the workbook.
~ save and close the workbook

now you should be able to open the workbook (with workbook protection on),
update pivot tables and run SaveAsCopy at will during the same open session
and not receive the "unable to read file" error the next time you open the
workbook.  

The other option is to shut off workbook protection, or at least toggle it
off before close so it's off when the workbook is opened again.  I chose not
to do this because I absolutely need my workbook protected and if you
unprotect the book in the BeforeClose event and the user cancels when asked
if they want to save changes, then the user remains in the workbook and the
workbook unprotect has already run.  No easy way to trigger the protection
back on again.  Much easier to shut off the Save Date With Table Layout
property on the pivot tables and leave protection alone.  

hope this helps.