I am trying to install MSL 10.5 on a HPDL360 Gen 9 server and am having all kinds of problems.
I have gone through the MSL Qualified list and enabled Legacy Bios Mode. I was eventually able to install MSL 10.1 from a USB stick but after install is done and the server reboots it will not boot from the Hard Drives.
I have also setup the Logical Array using RAID1 before starting the Install of MSL.
You can't do the RAID setup, the MSL installer uses it's own LVM management, that is why it fails. Break the RAID and disable it, then reinstall and it should work fine.
Did you check the release notes on Mitel Connect? It seems to me there are more changes necessary in the BIOS besides Secure Boot... When you look at the recommended server list and go to the notes after it, it usually specifies exactly what needs to be done.