Agree with Kashifme21. You might need to reinstall the firmware, because it got wiped out, when you accidentally pressed "format". This may include hacking the hard drive, which as said may include your XBOX 360 profile to get banned. But I dont see any other way of doing it, unless you try to send it to Microsoft, which probably ends up costing you a lot of money.
So I did some research on google and I found this page which might help you (Well Microsoft wont be happy for it)http://digiex.net/guides-reviews/console-guides/xbox-360-guides/3152-how-hack-250gb-320gb-sata-drive-work-xbox-360-xbox-360-slim.html
It will cost you to buy a Western Digital harddrive though, but it's cheaper than the XBOX 360 Harddrive