First off, are you sure all the photos were actually copied to the external drive? I know, sounds like a silly question, but it is valid. Also, when you copied them over, did you specify a location, like a specific folder? Lastly, and hopefully, you didn't delete the photos you tried to back up. Do you still have the originals on your computer? If so, try copying them over to the external again to see what happens.
Lost data is tough to recover even with the best tools unless you know the name of the files you're looking for. In the case of 400 photos, the only way to find them if you don't know the names is to do multiple searches either with recovery tools, or manually. This can take a very long time especially when you're hunting for specific file extensions like jpeg, tiff, gifs, pngs.