How it works should be in the manual. *GASP*
My Fusion has a hard drive in it, and I honestly forget the size. 10 GB, maybe. In my car, I can only rip songs from CDs. I can't load audio files onto it, and it can't extract audio files from a data CD - it can only record audio CDs. It can't even record off of the radio. Pretty worthless to me, and it's only 10 GB anyway. However, I can connect a hard drive (or iPod) to a USB port in my center arm rest, and it will play those files. Now, that has come in really handy.