mt80,
Well, first off we need a little more information about your box, i.e. what mod-chip do you currently have installed, what version is your box, hdd specs, 300 gig hdd dosnt really give us any clue as to what drive you are currently using... If you are using, the make and model would be nice.. :)
Furthermore, your current Bios should support it, if not try upgrading the BIOS. Flash ur chip with the latest version and that should fix ur problem. fyi, you can install just about any size drive on a softmod. I currently have a 300 gig westren digital in one of my softmods.
Good Luck! :beer: