http://boardsus.playstation.com/playstation/board/message?board.id=ps3&message.id=3318910
Ok to the HDD makers 320 GB means the drive can hold 320 billion bits of data. Except 320 billion bits does not = 320 GB worth of actual bits. It =298 GB. Now the PS3 reserves 12% of the HDD for it's own purposes. This is true no matter what size drive you use. So 298-12%=262 GB left assuming the drive was empty. Now you say that your back-up was 18 GB. So 262 GB -18 GB=244 GB.
la PS3 reserve de 10 a 12%