Moving files from one external hard drive to a new external hard drive

I am trying to help my son who is working a documentary on Premiere Pro. He had the first section of the documentary already mixed and  saved on an external hardrive but the hard drive had a problem with it so we got him a new hard drive. He copied all the files from the old hard drive to the new one. But now when he goes to play his project (the portion of the documentary that is already mixed) it won't play and says that the necessary video files are missing. Can anyone explain what he did wrong copying the files? Frustration levels are very high!