The FIDs are both the song information, AND the MP3 files. It is the source for all song database information including the playlist trees and what is in them. The entire details are here.

Edit: Let me be clear about this: You can't easily transfer the song information without also transferring the MP3 files, too. Once that stuff is on the player, the things are rather carefully intertwined and hard to separate. So you're in for a long wait no matter what you do. The difference here is that the wait is only to copy them in one direction (from the old hard disk to the new hard disk, as opposed to back to the PC then back onto the player).