http://support.microsoft.com/kb/937279?sd=xbox
Transfer content from your existing Xbox 360 Hard Drive
You have the option to perform a one-time transfer of all content from your existing Xbox 360 Hard Drive (source drive) to another Xbox 360 Hard Drive (destination drive). Whether you decide to do this or not, understand that both drives are fully compatible with your Xbox 360 console and will be after the transfer is completed.
Important Content can only ever be moved from a smaller source drive to a destination drive that is the same size or larger and only one time. For example, the following hard drive pairings allow transfer from source to (">") destination:
20GB > 20GB
20GB > 60GB
20GB > 120GB
60GB > 60GB
60GB > 120GB
120GB > 120GB
Note If you intend to move content from an existing hard drive, such as your 20GB hard drive, to a new destination hard drive, such as to a 60GB hard drive, do not use the destination hard drive before you perform this procedure or you will lose whatever content that you have previously put on that destination drive.