Connect the SSD´s to the SATA III ports instead of the SATA II ports.
The Z420 has four SATA II ports and two SATA III ports. I think HP messed this up with the Zx20 series since the boot drives in a new computer are attached to the slower SATA II port while the DVD burner are connected to the SATA III port. Really odd imo.
I may be wrong, but my gut feeling says that both your SSD´s are connected to two of the four SATA II ports at the bottom of the motherboard. Connect them to the two SATA III ports. The DVD burner in you computer is connected to one of the SATA III ports, so just follow the cable from the DVD burner.