A while back I started playing Gears 2 on insane difficulty. I got about halfway through before losing interest. Fast forward a few months/years, and my old Xbox E74'd, so I got one of the new slim models (yeah, the old one was out of warranty). Anyway, I plugged my old hard drive into my new machine and decided to finish the insane playthrough, just for the heck of it. I proceeded to do so. Oddly, however, I didn't get the achievement for beating the game on Insane.

...any idea why? Does it have something to do with having Gears data on both storage devices (I had played some random online games before putting in the old hard drive), or starting the playthrough on one machine yet finishing on another? I'm positive I played start to finish on Insane difficulty, so I'm not sure what else it could be.

I know it doesn't really matter, and I'm not a gamer-score-whore by any means (I have like, 5000ish), but it'd still be nice to have the achievement to show I got through Insane. Any ideas on the root cause would be appreciated.

EDIT: Yep, I checked and it's only giving me credit for what I've played on the new xbox. No completion credit at all (any difficulty) for the stuff I did on the old machine. This despite me starting on a save game that was halfway through. Blah.

EDIT 2: Screw it, I just played through the stuff it wasn't giving me credit for, and got the achievement. Still would like the questions above answered if anyone is feeling helpful.