I have done this one achievement about 60 Times On Alone in the Dark... AND It's the LAST ONE As well!!
But every time I turn the box off... I come on the next day - AND ITS NOT THERE! - I know its not the hard drive because I have just bought a new one - to replace my crippled one and my disk is in mint condition like all my disks :cool::D!

WHAT DO I DO!?

Does one achievement really bother you that much?

You know you've completed the game and can prove it by describing what happened.

To be honest I don't see why you need the confirmation from your 360 that it recognises that you've completed it. :)

I suppose - I just want my gamerscore up - this achievement is 50g

50g isn't that much really, for a single achievement maybe, but you might be better playing a game you find fun and completing that rather than doing the same bit of the same game over and over again and getting frustrated with it