IRiver
From Jon's Wiki
Not content with simply installing Rockbox to replace the crummy firmware, I want to pimp my H120, which currently has 20 GB of space, to 40 GB. To do this, apparently all I need to do is unscrew the casing and replace 1.8" hard disk with a bigger one; it's just an ATA6 connector. For 40 GB I will need to buy a Toshiba MK4007GAL from somewhere, or if I am willing to remove some rubber shock absorbers (probably a bad idea) I can get a two-platter Toshiba MK8007GAH which has 80 GB of space.