Dualboot failure - update : Grub fails to boot Windows RAID array

I have installed Win 8.1 on four Samsung 850PROs in RAID5 (OS as well as the Windows bootloader on the array).

When I started to install Mint 18 on the NVMe drive, it didn't see the Windows install. I let it install anyway, thinking that GRUB would sort it out afterwards.
After the installation, the PC failed to see ANY operating system and went straight into its EFI shell. Removing the NVMe drive allowed it to boot into Windows again. Re-installing the NVMe drive and disconnecting the Windows SSDs allowed it to go straight into Linux.

So it boots fine with either OS, but not at all when both are present at the same time.

When everything is connected and I go into the UEFI's boot menu, it only claims to see "Ubuntu". Pressing that option throws me straight into the EFI shell.

Specs :

MoBo : MSI X99A Gaming 7
CPU : i7-5930K @ stock speed
RAM : 32GB (4x 8GB) Kingston HyperX Savage DDR4-2666
GFX : 2x MSI GTX770 4GB
NVMe : Samsung 950 PRO 256GB
SSDs : 4x Samsung 850 PRO 256GB in RAID5, using motherboard's on-board RAID controller

I've installed Windows twice tonight, Linux multiple times. The first time I installed Linux while the Windows SSDs were connected. When that failed I wiped everything and installed each OS with only its own drive(s) connected. That didn't help either.

It dualbooted just fine before, when Windows was on the NVMe and I had Mint 17.3 on a single 850PRO. I had it running Legacy BIOS (never really saw the point of using UEFI really)

Should I try it again with legacy BIOS instead of UEFI or is it the RAID that's the culprit here? Not looking forward to installing everything yet again.

Good practice is to disconnect all drives but one you want the OS to installed too, this way there are no problems when modifying the boot, you can just select the drive from bootlist and your good to go.

I would also avoid using UEFI if your booting into more than one OS (I too don't see a need for UEFI) this makes the process much easier.

Normally I leave the Windows drive in the PC when installing Linux, so that GRUB picks it up right away. However when that failed last night I already did a fresh install of both OSes with only their own drive connected, to no avail.

Kinda not fun, having to remove the NVMe drive every time. it's wedged between 2 graphics cards.
Good thing I moved the bottom card down a slot yesterday, otherwise it would be covering the NVMe drive and I'd have to remove the card every single time.

With regards to UEFI, a bit more searching does indeed reveal that with UEFI there is no guarantee that it will boot at all when dualbooting.

Currently installing Windows (again), this time using Legacy BIOS. At this point I don't even need to look at the card anymore to enter the Windows key.

Well, we're getting somewhere. I can boot into Linux just fine, and using the boot menu I can boot into Windows.

However Grub looks like a no-go. It sees the Windows loader and I can add it as a startup entry.
When booting I am greeted by the Grub screen. From there I can go into Linux. However when I try to boot into Windows I get

error: no such device: 1C4CC3554CC327FA.
setting partition type to 0x83
error: invalid signature.

Press any key to continue...

When pressing any key, I go straight back into the grub menu.

I installed Gparted to see if the array's UUID might be the issue, and I'm getting a "Can't have partition outside the disk!" error. I assume this has to do with the Windows install being on a RAID array. The UUID and dev/wrapper path are correct though, so I'm stuck again.

Oh well, I suppose I could live with having to press F11 repeatedly whenever I need to be in Windows.

Yeah, you could set your boot media from the BIOS to run it on power on (If you want your windows or linux to boot on default), however you do need to choose a boot device when you want to change it via F11 (Mines F12). I find this ok since this reduces the complexity and issues extremely.

Plus when you install a new OS with only the intended blank media with the other media disconnected, this doesn't mess with any of your boot parameters and partitions, very clean.