Anyway, I copied all the data off it and on to my new hard disk. Everything seems to work fine except my Forza save. When I load up Forza my save game is there, but when I choose it, it starts to load then just hangs. I has similar problems in the past, and it was to do with changing the system timezone in the MS dash. I can't for the life of me remember what I did though.
I've also read that changing the system language from English to something else and back fixes it. That didn't work either.
Then I found the little program "ForzaSign" and figured that I might have to resign the save game to make it work from my new hard disk. Thing is, both disks seem to have the same HDDKey. Namely