You can turn on HBA mode on a HP DL380P G8 with the P420i RAID Controller but the OS won't recognize the drives and all sorts of weird things happens to the drives. Plus the RAID Controller and cache is disabled.
Here's how to enable/disable HBA mode:
Make a bootable Linux Mint USB. https://linuxmint.com/download.php
Boot it up and go download hpssacli-2.10-14.0_amd64.deb from here. https://downloads.linux.hpe.com/SDR/repo/mcp/pool/non-free/
Double click on the downloaded file in Linux Mint and install the program.
Open up your command prompt and go to: cd /opt/hp/hpssacli/bld
Then run the command: hpssacli
You will see two right arrows meaning it's waiting for a command.
To turn on HBA Mode Enter: controller slot=0 modify hbamode=on
To turn off HBA Mode Enter: controller slot=0 modify hbamode=off
If you turned HBA mode off, after booting the server, full RAID control will once again be available.
I tried HBA mode and installed TrueNAS Scale but the server would not boot into the OS. Better to leave HBA mode off and just load Windows Server or get an HBA card like a Fujitsu LSI 9211-8i, pop it in a Riser slot, hook up the cables to it and enjoy your JBOD's.