HALF-FIX found!
But another problem rises

Alright here goes: disable XMP
I bought two MSI x99s SLI PLUS, in part because I wanted to use the XP941 and it was recommended by MSI
One of the board strangely was not offering any XMP settings, on the first one when booting to UEFI it would display an XMP button, on the first board I activated it, on the second one it was just not available even if the exact same ram was used (Corsair vengeance DDR4 2800). When installing windows I noticed the XP941 was showing up, I was very happy and basically assumed the other one was defective.
The RAM thing kept being annoying me though and I noticed in MSI Intel extreme utility that when running the RAM stress test it would just fill 3112MB of memory rather than the whole 16GB, I thought maybe my RAM was defective in some way so I plugged the first workstation back and turned it on and ran the stress test on it again only to notice it would not fill the RAM on this system either. Happy that this was normal I rebooted disable XMP and entered 2666 as the speed leaving the processor at base value, this way both station would be identical.
That's where the magic happened, the XP941 suddenly showed up in the disk management utility and I was able to format it.
I still cannot booth from it though but as far as my needs are concerned using it as a data drive is all I wanted.
the 2nd problem remain; why is one board offering me an XMP option and not the other, should I worry about having defective RAM? (BIOS version are identical 1.10, the latest).