Laura,

Try this in your external data sheet. Insert a column to the right of
your identifying column. In this case I used A as the Company
identifier.

=IF(ISERROR(INDEX(Internal!$A$1:$A$11,MATCH(extern al!A3,Internal!$A$1:$A$11,0))),"Not
Found", "Found")

This will put Found or Not Found based on your list of internal id's.
If you want, you can then conditionally format the identifiers to color
the row if Not Found is populated. Select the entire row and
FormatConditional Format

Formula is: =$B2="Not Found", select your formatting. Use the format
painter to copy this down all of your rows making it easier to identify
the missing data.

HTH

Steve


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