Boot From the Thumb Drive and Format the Hard Drive
Afterward, boot from your DOS thumb drive (specify a boot drive in your PC's BIOS, if necessary). At the DOS command line, type "run hddhackr –f" (don't include the quotation marks) and press Enter. You'll be prompted to make a backup of the original drive firmware; do so. Continue with the process to flash the firmware on the new hard drive. After a few seconds, you'll be prompted to restart the computer. Once you have done so, run the "hddhackr –f" command again, and it should report that the hard drive has been formatted for the Xbox 360.