I've created a new book, and copied the data from another wookbook
which contains new data each day.
e.g. =C:\[new_data.xls]Sheet1'!C15

After closing all the files, upon reopening my new book I get an
error, "Unable to read file". When i manually open the new_data.xls
sheet, my book then updates the data.

Can anyone suggest how or why this might be happening?