Is it possible that the files are hidden? Go to Tools-->Folder Options-->View and select Show Hidden Files and Folders and uncheck Hide Protected Operating System Files
I don't have any idea why they would be hidden - what kind of files are they?
Also, have you tried to "Safely Remove Hardware," unplug the drive, and then plug the drive back in and see if the files show up then? Windows can be buggy like that sometimes. You could also try opening command prompt (Start-->Run-->type 'cmd' ,<enter> and type the drive letter for the External HDD (if the drive is 'G:' , just type 'G:', <enter> then type 'dir' to see the directory name and size. Then type 'cd\directoryName' , <enter> and type 'dir' to see if you can see the files that way.