Invisible files on external hard drive

[bobcorey]

Hello I have a macbook pro 13 and an external had disc. It runs fine until the following issue appeared: I use it to backup my files (folders with wav files) from my macbook. I copied some of them into an other folder of my external disc's folders (that seems to "eat" my files). The copy completed but when I went back to that folder there was nothing inside (there wasn't any problem while pasting). Same happened with a lot of other folders that I copied into that exact folder in the past. It still says that this folder is empty. How could I find these files?
P.s. I plugged the external drive to a PC but nothing was shown in that folder either.