You can take out the tv tuner and put a hard drive in there, it's just a regular spindle like those used for any drive, but it voids the warranty if I remember correctly. I would ask sager if they could do it for you (and if anyone is interested in buying one with this setup, I recommend asking about it as well; havin them do it would prolly be the only feasable way).
Also, proteron, you should consider that the 8 megs cache on the hard drive won't make a lot of difference when on a firewire connection. If you haven't actually bought the drive yet, I would recommend looking for one with less cache if it is available just to save yourself some money (or get a bigger one for the same price, etc). I don't know how feasable this option is for you, but I thought I'd let you know anyway.