That link is for a Xbox with a custom BIOS (modchip of TSOP flash). You only have the Rocky5 softmod installed or also a custom BIOS (like Cerbios)?
If you only have the softmod, you'll need the eeprom.bin which the softmod installer made to lock the new hard drive. This locking and formatting can be done with xboxhdm23usb or FatXplorer with the new HDD connected to the PC using a USB adapter.
If you already have a custom BIOS, you can just format the new HDD with FatXplorer, put the dashbard files on the C and install the drive in the Xbox. No need for TruHeXEn.