The system does come with the M.2 drive running as SATA and the controller set to RAID. I don't know anything about the Micron drive, but a RAID system should pick up the 2.5 inch drive.
My system is currently running with SATA set to AHCI because I have installed a Samsung M.2 drive. A Samsung 2.5 inch drive was recognized as expected.
If the drive is not recognized in the Bios, check the connections A RAID ready system should see the second drive. However, I cannot guarantee your configuration is allowed since mine is different.
The problem with running the Toshiba drive as PCIe is the Windows NVMe driver. The drive performs better as SATA. The M.2 slot can only run a PCIe drive as SATA if the SATA controller is set to RAID. If you want to change the controller, it can be done by using msconfig.exe process. Let us know if you want to try that.
@Saltgrass - what two drives do you have in your system?
I have a Samsung 960 EVO and Samsung 850 Pro.
I installed the 850 (2.5 inch) after I had the 960 set up a primary drive. Since the system was already set to AHCI, the second SSD installed normally as a Data Drive.
Strange, your drives are essentially the same type as mine.
To switch to AHCI, I did the msconfig/safe mode/ahci/undo safe mode... plugged in the 2.5" drive.
The Toshiba is seen as a M.2 PCIE drive in AHCI mode. But the Micron BX100 (SATA ) is nowhere to be seen in Windows device manager (and obviously not in Disk Manager). I did confirm that both drives are seen in the bios.
There is really nothing else to do right?
Crucial states the BX100 is a legacy drive and no longer available. Perhaps it is not compatible with Win 10.
All I might suggest, if possible, is to put the drive in another system and use Diskpart to clean it. Maybe that would help it get recognized correctly. If you don't have another system, perhaps an external enclosure..
To bring closure to this issue, I also tried a Samsung 850 EVO in the 2.5" bay and it is recognized both in the bios and windows 10 (raid mode). No fuss whatsoever. I guess the BX100 is just incompatible. Thanks Saltgrass.