had a look there, there's a couple of threads but nothing particularly conclusive that i could see. ideally i'd like a "i used a seagate 320GB drive and it's great" type of thread and all i could find were discussions that were a bit inconclusive. i'll join anyway and post though, ta.
You could buy a kit from a company that specialises in this sort of thing. Then if the disk is not suitable you could probably return it to them as it would not be fit for purpose. Top Google return was xtendedplay.co.uk (http://www.xtendedplay.co.uk/humax-drive-upgrades-ct-3_18.html). I have not bought from this company and am not affiliated with them (though I am tempted by the 500GB upgrade ... Baseball on Five recordings run to 7GB each, i.e. 14GB per week!).