iTunes external drive question

I just moved my iTunes library from my PC's drive to an external drive. I ran into a problem whereby all the files that were extant in my library give me the error message "could not be used because the original file could not be found" when I try to play them.

I can locate the files in the external drive and open them one by one, but is there a simple way to do it for all the files at once? Thank you.