@JOcean @jphughan 

Dell support article How Bootable Devices Work in UEFI Boot Mode does not include dual SSD start-up boot menu topic. Where can this Dell information be found? 

@crimsom  Normally you would install a bootloader application that supports multi-boot environments.  There are multiple options there, including Windows Boot Loader.  I haven't kept up with alternatives since I just don't do multi-boot often.  But In that case, at a system firmware level, you always boot from a specific device, namely the device (and more precisely its partition) that contains that multi-boot bootloader with whatever boot options that you have configured.  And then THAT might turn around and boot from a different device depending on your selection.

If you want to try to set up fully independent multi-boot, then you'd probably have to add a boot option for each device's bootloader separately.  The default bootloader path according to the UEFI spec is \EFI\Boot\Bootx64.efi, but the path to Windows Boot Loader when installed on a hard drive (as opposed to bootable Windows installation media) is different.  That path would exist on the small, FAT32 EFI partition of each disk.  If you want to do that, then you'd just manually add a boot option to your BIOS Setup for the environment on each disk.  But in that case, you have to change which disk the system boots from at a system level each time.  The easiest way to do that is to press F12 during startup to access the one-time boot menu every time you want to boot from whichever environment isn't your top ranked, but that's still a bit inconvenient -- which is why the first option I described is more typical, i.e. the system always loads your multi-boot bootloader from the single device that contains it, and then that knows about your various environments and automatically gives you the option to boot from them each time.

I realize that's pretty general guidance, but that's all I can offer here since this isn't something I work with on a regular basis.  Hopefully it helps!

Hi @jphughan  thank you for your contribution. Not all computer configurations are alike, even when they are from the same Dell brand. We have visited our 17 R5 UEFI setup page and dual booting from different Solid State Devices (SDD) is not obvious. There are very generalised guides such as BIOS/UEFI Setup Guide. Our search to find the Dell specific answer continues, in the belief that we are not the first to have asked this question. 

@JOcean @jphughan 

The 17 R5 was delivered with its bootable M.2 solid-state drive in the SSD-3 slot. It was moved and installed in the SSD-2 slot as shown in the following image. 

When the 17 R5 is powered on, the >>>Checking Media Processes... window appears followed by a SupportAssist window that says SupportAssist is running a system scan to detect any potential hardware problems with option to press F2 for BIOS Setup. Having entered BIOS setup, we see that the First HDD and Second HDD entry is now declared as None. The Third HDD entry continues to acknowledge our storage HDD.

We were expecting BIOS/UEFI to automatically detect the M.2 solid-state drive location change from the SSD-3 slot to the SSD-2 slot, but it did not and we do not know why.

The 17 R5 BIOS/UEFI v1.8.1 is not password protected and pressing the down arrow gives access to change system time and system date, but nothing else. So we cannot manually update the First and Second HDD entry. We are also puzzled that the M.2 PCIe SSD-1, SSD-2 and SSD-3 entries all say None. 

We now return the M.2 solid-state drive location to the SSD-3 slot and BIOS/UEFI Second HDD acknowledges the bootable solid-state drive, and the 17 R5 boot function is fully restored. 

Can someone please tell us what the 17 R5 BIOS/UEFI setup should be, why the HDD entries are being used, when there are M.2 PCIe entries. 

There are a couple of possibilities:

First, that the "first HDD slot" doesn't support SATA (that it's NVMe only).  

Second, that the "first HDD slot" is faulty.

Assuming the system came with the SSD in question, was it in the "second hard drive" slot initially?  If so, and the other one was left empty, it's likely the first scenario holds.

Do you have an NVMe drive you can use to test that first HDD slot?

The following BIOS/UEFI image is the when M.2 solid-state drive is in the SSD-2 slot. 17 R5 does not boot.

The following BIOS/UEFI image is the when M.2 solid-state drive is in the SSD-3 slot. 17 R5 boots as expected.

Hi @ejn63  thank you for your contribution. The generic BIOS/UEFI accommodates three HDD and three M.2 PCIe solid-state drive entries. Whereas the 17 R5 supports three solid-state drives and one hard drive. The following image shows One M.2 2242 solid-state drive slot (SSD-1) and Two M.2 2280 solid-state drive slots (SSD-2 & SSD-3). These have interface type SATA AHCI 6 Gbps, PCIe NVMe up to 32 Gbps and Capacity up to 1TB. 

We have two bootable M.2 SSDs complete with their operating system and both have been successfully used in the SSD-3 slot. This is the original BIOS/UEFI setup. When the bootable M.2 SSD is put in the SSD-2 slot, the 17 R5 does not boot. We do not know how to tell the BIOS/UEFI the order in which to look for the bootable M.2 SSD on the 17 R5 System Board. Our first problem is the activate the SSD-2 slot function in the BIOS/UEFI setup. 

The system may be hardwired so it won't boot other than from the M.2 socket originally equipped with the SSD.

That said, it's more troubling that the second socket doesn't even show a drive connected.

Hi @ejn63  both SSDs are M.2 PCIe SATA 2280 SK hynix HFS256G39TNF-N2A0A. These came with the new Alienware 17 R5 Laptop. They are not M.2 PCIe NVME 2280 Solid State Drives as listed in Parts & Upgrades for your Alienware 17 R5. 

Is your SATA operation set to RAID or AHCI?

SATA Operation is [AHCI]. 

The bootable SSD cards have different Windows 10 Pro Operating Systems, they are not clones. 

Hi @ejn63   the 17 R5 BIOS/UEFI is v1.8.1 and we note that Dell BIOS/UEFI v1.11.0 is responding to Intel Security Advisory, and Intel Management Engine Components Installer v2014.14.0.1540, A04 installs Intel Management Engine Interface Driver and Intel Management and Security Status software.

Now the BIOS/UEFI is seemingly locked.

We do not know why the SSD-2 socket is not functioning.  We have not been told that our USD $3,000 purchase has been hardwired so we can only use the SSD-3 socket for a bootable Solid-State Drive. Our Alienware 17 R5 Laptop is within its one year RTB warranty, but Dell UK / NI have turned their back on our System Service Tag. 

If you have two M.2 SSDs, neither of which is seen when connected to the same M.2 slot -- I'd say that's relatively substantial evidence that slot isn't working, particularly if the system will boot from the other slot but never see a drive mounted in it.

Hi @ejn63  we were sort of with you, until the last part.

When one or other bootable M.2 PCIe SATA SSD card is in the SSD-3 slot, the BIOS/UEFI acknowledges it in the Second HDD entry. 

Yes, that much has been clear all along.  It's what you've labeled the "second SSD slot" that's suspect.

It's also very possible that the system will support only two SATA drives -- you might want to try removing or disconnecting the WD spinner from the system to see if that allows  you to see the M.2 SATA in the second SSD slot (it'd be a good idea to cease referring to it as a PCIe SATA drive, and just refer to it as SATA, as it's either PCIe NVMe or SATA -- not PCIe SATA!).

If you CAN see both SATA M.2 drives at that point, you'll know the system has only two SATA drive support.  You likely can still have two M.2s in the system plus that spinning SATA drive -- but to do so at least one of those SSDs will need to be PCIe (NVMe), not SATA.