The ps3 is very tempremental with s**t like this, you need to do the following.
Open the External HDD on your computer and make 4 folders, PS3, PICTURES, MUSIC, VIDEO, all in capitals. This has to be done so the ps3 can recognise the folders. When you turn your ps3 on you should get an notice on your pc saying "media server found" or something like that. If not, go to pictures on your ps3 and search for a media server, you should be able to find your pc. Then its a case of looking through all your media on your pc via the ps3.
Alternatively you could try and plug the HDD into the ps3, hover over it, press Triangle and select "all media" (or something like that). That will show all the media files on the disk that the console can read.
PC gamers are isolated social degenerate peados -Reegee