1024 Bytes (not 1000) in a Megabyte prob. explains why you're reading only 15GB.
I love my Rockboxed CF Mod IRiver. Much lighter, quieter, way better battery performance. I remember some struggle and drama when I initially did the CF Mod. But when I get one of those Sony M10s, not sure I'll use it anymore.
Rockbox is not just for IRivers though......
I Rockboxed an IPod Video 60GB, loaded it up with live FLACs, it plays DOOM, etc. Totally cool, especially once you get a "theme" together which shows "album art" (read: pictures of Jerry playing live) track lists, etc. I've only got about 10GB left (though keep pruning it) so I think my wife's old Toshiba 60GB Gigabeat is next.....
What would be totally cool is dropping a 1 TB drive in one of these, but I don't think they make em that small.