unRAID Boot Issue - MSI/SAS Card USB Conflict?

unRAID 5.0.6 Plus license on Lexar Firefly USB stick

MSI P7N SLI Platinum MB

Supermicro AOC-SASLP-MV8 SAS Card

As we are approaching the capacity of our current unRAID server I decided that it was time to increase both physical capacity (bigger case for more drives) and CPU. So I managed to get hold of a HAF Stacker case and a pre-populated MSI P7N SLI Platinum MB. The current working set up is a Gigabyte GA-MA78GPM-DS2H running an AMD AM2+ processor and a single SuperMicro SAS card (AOC-SASLP-MV8) running 6 disks on a Plus license.

The new MSI board boots fine from the USB key when the SAS card isn't installed. As soon as the SAS card is in place all I get is a black screen with a single blinking "_" cursor. I don't even get an error requesting that I add a suitable boot drive… just a blinking cursor. It makes no difference to the number of drives connected either to the SAS or the MB. As soon as the SAS is plugged in, booting fails.

Other versions of the same SAS card do same thing and the card is working - the correct HDD show up in the card BIOS. I'm at work, but I think the SAS card is still version .15 - I've never updated it.

I've been through the Lime-Tech as well as various other linux forums trying to solve this issue without any success. I'm hoping that someone here might have other suggestions to help me solve this.

I've tried the following - all result in the same blinking cursor:

• The correct (named) USB device shows up in the BIOS and is set to be the primary boot device.
  
• USB device is set to be the ONLY boot device - disabled all other drives
  
• Tried telling the system NOT to boot from any other device
  
• F11 Boot menu & select the USB stick from the options
  
• The USB stick boots fine in an alternative PC. Checksum passes with no errors in a Windows machine. Make bootable re-done and still boots without SAS or in another machine.
  
• I've also done a fresh Install of unRAID 5.0.6 on another (known to work) USB key with same results.
   
  
• SAS card has the 'INT13' option disabled (didn't previously on my old Gigabyte board, but is now). RAID mode/option is set to be JBOD on the card BIOS.
  
• SAS card tried in both PCI-e x8 slots.
  

The HDD attached via the SAS card all show up in the MB BIOS - I have also tried booting with them listed (but low priority) and with them disabled via the MB BIOS boot list

• Tried limiting the visible/usable devices in the boot list by disabling them via MB bios.
  
• Turned off IDE drives & e-SATA drives - even though nothing connected. I've also disabled on-board audio and 1384 ports.
  

If I use a HDD that has a working OS on it (OpenELEC), I can boot. That is true if the HDD is connected via the MB or the SAS card.

I've also reset the MB BIOS back to the default just in case I've done something stupid in the early hours while trying to solve this.

Going quietly insane here, every suggestion I can find on the forum seems to indicate that the issue either lies with the SAS card 'INT13' boot hijack or that the USB itself isn't bootable, but nothing suggested has worked for me. I'm starting to suspect that the MSI board just doesn't want to play nice with the SAS card…..

Any other suggestions before I try to offload the MSI board and have to explain to the finance committee why I needed to spend stupid cash on a new motherboard, RAM & CPU….