Dell Precision Mobile 5690 Documentation

Your Precision 5690 has two M.2 SSD slots, but only one is accessible for general use, while the other is used for a RAID configuration or a different purpose. 

RAID Configuration:
Some Precision Mobile 5690 models are configured with two M.2 slots, one for the primary operating system drive, and the other for a RAID array, which is a storage redundancy system. 
High-Capacity Battery:
The location of the second M.2 slot might be underneath a high-capacity battery, making it inaccessible or only usable for a specific purpose like RAID. 
Bifurcation Support:
If you are using a PCIe adapter to connect two M.2 SSDs, it could be that your motherboard does not have bifurcation support, which is required to split a single PCIe slot into multiple lanes. 
Intel RST Technology:
The Intel RST (Rapid Storage Technology) technology might be controlling the RAID configuration, and it might only support a specific type of SSD or a specific slot for RAID. 
BIOS Settings:
You might need to enable RAID support in the BIOS settings, or disable VMD settings to change the storage configuration. 

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The system board has two M.2 slots for SSD1 and SSD2, number 11 and number 6.

https://www.dell.com/support/manuals/en-ae/precision-16-5690-laptop/precision-5690_owners_manual/removing-the-system-board?guid=guid-dca4b2c3-b864-4d6f-be3c-9d7223bee82b&lang=en-us

@DELL-Jesse L​ 

Thanks...I just wondered why that other document said there was only one slot when I knew there were two.

Mine has two SSDs and is set to RAID. I'm going to upgrade them both to 1TB each this week (from 512GB) and restore the image.