What is the best method to validate a string's existance in a CLOSED
workbook that has many filled cells?

I need to check for a string in column A of an external closed book.
Column A of the external book has over 47,000 filled cells. I have a
VBA routine that writes a match function into a cell of thisworkbook,
but I get the error alert, "Excel cannot complete this task with
available resources..." when the external book is closed. However, it
works fine when it's open.

I've tried both the countif and match function but both fail when the
external book is closed.

(Excel 2003)

Thank you for your help.
-Chuck