I had to upgrade hard drives as I had a 20 gig and was quickly running out of space. The issues you're having is that save games don't save to the profile, so what you completed on one hard drive isn't saved to the profile when you use another hard drive. Microsoft sells transfer kits (as many consumers have had to purchase new consoles and hard drives) that allow you swap all of your saved info to a new system. The problem with these is that they're around 20 bucks and you're only gonna use it once. My advice would be to look around the used market, as many people sell them for cheap once they've finished using it (I gave mine to a friend). The device consists of a cord that connects from one HD to the other and a dvd you put in your console. I know you already beat it on insane (and congrats, I'm quite the gears fan and have yet to accomplish this) and you probably no longer have your previous HD, but for future reference...

I also occasionally throw saved game files on my memory card and move them to other profiles, should I go to a friends house and want to continue with my game. Putting your profile on a jump drive/memory card can be beneficial too, as you won't have to upload the profile (20 min process). Just plug it in and you're good to go. Also, if you weren't on live when you played Gears before, then when you moved your profile to a new console, all of that information will be lost, as you weren't online when you played, so it didn't update your progress towards the achievement (confusing, I know). In short, Microsoft needs to uncomplicate the use of profiles by a lot...

also, I'm K1NG G3EDORAH on live if you'd like a fellow gears competitor/teammate. Hope that helped some...