While there are reasons for the other 30 GB to be missing this post does not offer details about the old and new machine, OS in use on old and new and the adapters and OS you are using.
With all those details left out all I can offer is that you check the LOST AND FOUND post at the top of this forum.
It's at http://forums.cnet.com/7723-7588_102-354911/lost-and-found-or-the-storage-forum-sticky/?tag=contentBody;threadListing
Okay so a while ago, an old computer crashed and I had to restore everything. The hard drive on it was a 40 gb. When I got it working again, it said I only had a 10 gb hard drive, so obviously the data is still on there somewhere, but I cannot find any way to access it. Did I create a partition or something?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic