Try VLC to play it - that does most everything, re: transfer, what message does it give you?? It could be that the external HD is not a compatible Mac format. A bit more info is needed if you want help.
If they don't show on the desktop then they must be some strange format - if you are using a home dvd recorder have you finalised the disks on your recorder - they will play on the recorder if they are not finalised but won't play on a dvd player (or show on your Mac)