use the equivilant of "dd" to big (external) SATA Drive
//Try to have it reformat from xbox to reclaim more space;
// the reclaim space worked
//the reclaim space failed, but you have a HD working at 20gbs
celebrate at least you got a generic HD working;
puch MS in the kidney;
Theory behind this is as follows. If you format a disk from the XBOX 360, I think it perseves a partion of it that is needed. This is why we would write a straight image to the SATA (External) drive, even thought it would only see the 20gb, in theory, a partion in this image would hold the potentially required files to allow it to work.
That way, when you reformat it, it should "see" the unallocated space, and use it.
There is only one thing that could make this not work, is if the drives had special firmware. Good luck