HP DL380p Gen 8 - TrueNAS

Anyone have any success using the SATA optical port for an SSD?
The reason is for TrueNAS, you should put the SmartArray 420 into HBA mode. When you do that, you can’t boot the system off any of the internal drives connected to it. So you need another boot device to get the OS a off the ground.

My HP 360e G8 works fine with the 420 in HBA mode and booting from an internal SATA drive.

But do you have an SD card slot on that mainboard? You could also boot from that device though there are some small pitfalls to using an SD card for Truenas boot. Log files being the biggest as they get written to the boot drive by default.