1) Used Xport 360 to make an image backup of my original 20gig HDD.
2) Used HDDHackr to install 120gig SS to new WD Bevs drive
3) Used Xport 360 to do a FULL image restore of the 20gig bin image to the 120 gig drive. (Ignore every guide out there that tells you to not todo this) Yes, this will make the drive 20gigs, but only temporarily.
4) Use HDDHackr a second time, and once again apply the 120gig SS to the WD Bevs. This will corrupt the last partition with the 360 save data, but the other partitions including the backwards compatibility section will function 100%
5) Connect WD Bevs to Xbox 360, and use the System menu to format the HDD. This will ONLY format the last partition with the 360 content. It will not touch anything else.
6) Connect the WD Bevs to your Windows machine again. Manually extract all your 360 content from your 20gig bin image, then manually inject it back into your now freshly formatted partition of the new WD Bevs. Alternatively, if you have it, use the official M$ 360 Transfer Kit to transfer from the 20gig to the 120gig at this stage, since it only transfers that one partition also. This saves some time since it transfer the data in one shot.
Yes you have to jump through hoops, but it does solve the problem. Hope these instructions were clear.