You have to LOCK your new drive to boot it with the original MS BIOS.
TO make a backup, just put in your original HD (this one will boot with modchip OFF) and BOOT with an Evolution DVD or CDRW. (Netwerokcable attached !)
FTP to your XBOX and copy all drives to your PC.
Replace the HD with the BIG one and do the same and copy all file back to the new DRive.
(PS. Read the tutorials