you can just search for the "120GB hddss.bin" on google and download the file to enable your WD drive to work on your 360...then transfer all your game saves and stuff off of your old drive and then swap out the drives......heres a link to a tutorial on it..
http://forums.xbox-scene.com/index.php?showtopic=660096..I just bought a broken 20GB drive cheap on eBay..to get the HD case and bought a new 120 WD BEVS HD and thats what i'm gunna do..although i don't need to transfer any games save or whatnot so i can avoid that step.....good luck.