This is how you can put you'r old xbox 360 HDD into your new xboc 360 slim hope it helped and be careful when your doing this.
Note this is not my video
1.Take your HDD from old xbox 360.
2. Take apart the HDD and all the screws you should up with this. (be careful on this part)
4. Take the external HDD as in the PIC above and very carefully align it with the port's that it should go into. (this part is very hard sometimes and please be careful on this part to)
5. You should feel it and hear a "click" noise of sorts and then you should know its in.
here is a video in case i did not explain it that good.
I hope this helped you in some way
Note i am not responsible for anything that happens to your xbox 360 or HDD that happens durring any of these processes.