The ipod has a "files" mode which you can use to move files about. But these files, even when a MP3 will not play.
What does play is what's in the itunes collection. And what's on the collection does not have to be on the player and vice versa... At first this may confuse some.
The iriver on the other hand can have MP3 files and play such.
Does that help?
I'm a little behind the buying curve here, but.. I heard that with the i-pod, you have to keep your music on your computer's hard drive. If you delete the music off your hard drive, it then gets deleted off the i-pod the next time you hook the i-pod up. Is that true? What about the comparable i-river? I want to get something that can act as an extra hard drive for my music so that I dont' have to store any music on my pathetic 10GB laptop. what machine would be best? thanks!