The picture abowe is not correct.
The Z820´s SATA ports are: The two grey ports to the right are 6.0 Gb/s while the four white to the left are 3.0 Gb/s.
The SSD (boot drive) should be connected to one of the grey ports (6.0 Gb/s) since those two ports are the fastest one. I don´t know why the Z820´s were delivered from HP with the CD/DVD drive attached to a 6.0 Gb/s port while the boot drive is attached by default to the slower 3.0 Gb/s port.
I always changed the connections for the boot drive on every Z820 i sold when i was a reseller. I don´t remember if i had to enter the BIOS to change the boot order.