· Do I need to know Linux to use the Rio Car player?
No, you don't need any special knowledge to use the player.
Although the player runs the Linux operating system internally, you don't need to know anything about Linux to use it. In fact, the connection software is Windows-based.
If you happen to prefer Linux, there is also Linux-based connection software available. You can even connect with a Macintosh and load your tunes that way, using a Java-based utility.
If you want to write third-party software for the player, or you want to hack the player's internals, then you will need to know something about Linux. But this is not required.