Will I lose my achievements?

Ok so last summer, I got like 3000 points in achievements. Well since my xbox was all jacked up, it wouldnt let me go online, but I was still playing on my gamertag name. So Then I went to play on my brothers xbox, and I had some data on his hardrive. Well when I signed on I had only 9000 gamerpoints in stead of the 12000 I should have had. SO I later went onto my xbox, and I still had 12000. But I just recently got a new xbox the slimmer one, and apparently it has a hard drive in it. I recovered my gamertag, and I don't have any of the achievements I earned during the summer. Is there any way I can transfer my older achievements on my old hard drive onto my new xbox? PLEASE TELL ME I CAN GET MY ACHIEVEMENTS BACK! just to be clear, I can always just go onto my old xbox and my achievements will still be on there.