HPE Proliant DL380 Gen9 + GTX 1080 TI

[CsSerb]

Hi guys... I am owner of a proliant dl380 gen9 server with the dual zeon cpus and SAS raid config
I want to install a custom 1080 ti graphics card from asus outside my server externally with a PCIe 50cm riser cable and i got the 3x 8x slot first riser card that was installed by default... I don't have second 16x riser that requires second CPU.... BUT i have dual zeon cpus installed with 8 SAS drives in Raid 1 with p440ar controller and my BIOS is updated to 2.24(most recent one) set to UEFI by default and running stock hp win server 2012 r2...

My first question is : because of p440ar controller that can run in both uefi and legacy bios , can i swith my bios to legacy without destroying my raid 1 setup because i have important data on my drives ...? I know i need to install a fresh copy of win server after changing to legacy... But Just wanted to know if this will have an impact on my raid config or not...

And the second question is : i have asus gtx 1080ti strix o11g , a 50cm Pcie16x riser cable and external coolermaster 750m PSU for external duel 8pin power of the gpu ... Every time i try to run gpu connected to pcie 8x first slot . it will power on the gpu but never detecting it and no devices in device manager....
But
When i try to run it off the second 8x slot on the riser , on the boot check process (before bios check) after switching primery video output it gives me error with the title of : X64 exception type 2. NMI detected ... Sometimes boot will pass this screen without errors and give me post on second monitor but in the loading of windows it will crash and restart the system (i tried starting without matrox 200eh internal gpu and with only one monitor connected to gtx card but still no luck)...
And 1 another thing is when i get x64 error server second indicator LED starts to blink in red (the one with voltage pulse mark on it , under the start button). ...
I think the x64 error is because of the unstable signal from the riser cable and no ground wire from gpu to rack (correct me if I'm wrong)
And crashing is because of the bios that runs in uefi and i saw people running 4x 1080 gpus externally with riser cables with external power with no problems... My question is changing bios to LEGACY will be the end of the crash ? In case you ask ... YES i tried disabling secure boot or uefi enhancement option but still got the same problem...
Thanks