the z620 sata ports do not really care what order a device is attached. the primary boot device can be on any port and work reguardless where the drive is located. you only need the pwr/data cables to be able to reach the device(s)
however, most people (and HP) start with the lowest number sata port for the DVD, then first (boot) drive followed by second/third (data ) drives
the HP service manual for the z620 has quite a bit of dtail on the system, you might want to read it
use this sites search box and search for z620 service manual , or google for z620 service manual