This device is the one which Microsoft give to users with the elite console, but unfortunately not with the retail 60GB and 120GB HDD units. It allows you to connect a second X360 HDD to the back of the console via USB and effectively move your data onto the new drive.
This version of the kit will also allow you to connect your Xbox 360 HDD to your PC and transfer data to/from the HDD from there. Seperate software is required for this but is available as freeware online.
To use this is as a transfer kit with a 360 console then you will also need the transfer disc which is sold seperately. Beware of buying it from foreign buyers on Ebay as the disc is region locked.
The only other way to get one of these kits is to buy it from Microsoft, who will require the serial numbers from both of your drives before they will sell one to you or from sellers on Ebay, however they usually go for silly prices on there.