Updating linked files
 

I have Excel 2003 workbooks that are linked. The files are located in the
same directory but I do take them home and work there with the files on a
USB jump drive. When I copy them back to the work PC and open one, it asks
me if I want to update. When I click update all my vlookups and such return
#values. When I open the parent? file, the values are updated. Is this
normal that all files need to be open in Excel to do the update or is the
copying back and forth messing with something?
Thanks,

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Lee Coleman