I see, I did some googling but have not found a definate answer why this space is reserved, most likely is firmware installing and caching but what I find strange is that it uses a percentage intead of a fixed size, with a 320GB drive the reserved space will most likely never be used fully.Tragically the PS3 reserves a percentage of the hdd space. I was confused myself, I've lost the orginal article but having a quick Google, on a 320gb drive you end up with 263gb of 298gb free, which fits with your numbers.
Oh well, time to fill the drive with video and pictures and MP3's
