You Need the Western Digital Scorpio Blue, that brand is known to work. But its rumoured you cant go higher that 250gb .
WRONG, again. not rumoured, IMPOSSIBLE, you can only replicate hdd sizes that MS have already released and at the moment the largest size they have released is 250gb.
@raf i am now going to try and answer your origional question and end this topic.
ok so your options are...
-you can use hddhackr and use a laptop hdd
-you can try and find an official hdd without the case (unlikely to find one)
-put your 20gb hdd in the 120gb case
-or just go and buy a complete black hdd