Yes, if your external drive connects via usb then u can just plug it into one of the usb ports on the denon. As long as it is in mp3, flac, or one of the other compatible formats listed in the denon manual then you can just go to "net/usb" on the denon and play your songs.
I played a flac file off of a 1 gig usb flash drive once, so I don't see how a larger hard drive would be any different.
If thje hard drive does not have a usb connection then you will have to stream it to the denon from your computer with the hard drive with an ethernet cable or wireless bridge
