Also, I did get my 360 off of eBay and I know about the problem with some HD's that are sold over eBay being hacked and therefore no longer support backwards compatibility.
However, when I look in the system settings and then memory, the "microsoft" logo is present on my HD, and to my knowledge this means that it is an "official" microsoft HD.
For reference: Click Here
[Edited on 09.20.2010 7:57 PM PDT]