This should be possible to do. Load the 20GB image in xplorer360 using Drive->Open->Image, and extract everything under /content/<profileID> on partition 3 to a directory on your PC. Do Drive->Close, then Drive->Open->Harddrive/Memcard to connect to your 120GB drive, browse to partition 3, and insert the directories you just extracted from the image onto the new drive.
Figuring out your profileID isn't real easy, especially if you have a bunch of IDs on your old 20GB drive. What I did was recover my Live ID on the 120GB drive, played one game and made one save, then connected it to the PC. That way there is a single profileID (besides the generic 000... ID) under /content on partition 3 on the 120GB drive, and I just copied all directories under that same ID from the 20GB drive to the 120GB drive.
Note that after I did this I had to recover my ID a 2nd time on the 120GB drive, but I still had all my saves after I did that. At least it looked like I did, the couple games I tried worked fine.
XBOX BC saves are under /Compatibility/Xbox1/<something I can't remember>, so if you copy the stuff under there you should get your bc saves as well. I had some errors reading the files under that path on my 20GB drive, so I was only able to copy some of the files. As long as get everything under UDATA, I think you'll be ok.
This post has been edited by JBark: Nov 13 2007, 07:35 PM