XPS 8920, HDD boot priority

You need to hit F12 during boot up to select the 3rd HDD from the list of boot devices. 

SYou can change the boot priority to any bootable drive or partition by right-clicking on the This PC icon on the desktop and choosing Properties at the bottom of the context menu. 

(If the This PC icon is not showing on your desktop, type "This PC" in Search/Cortana.  "This PC" is listed in the column on the left of the window that opens.  Right-click on that and choose Properties.)

Choose Advanced System Settings in the column on the left.  On the System Properties page that opens, choose the Advanced tab and then the Settings button for Startup and Recovery.   In the top portion of the Advanced  tab page is a drop-down menu for Default Operating System.  Use the drop-down to set which OS you want to start the computer with.  It is a good idea to give meaningful names to each of your OS choices.  I use EasyBCD  (search online) to do this.  You can also conveniently set the default OS in that tool as well.  That is actually what I use all the time for multi-boot systems.

Don't assume "RAID" necessarily means some sort of array.  RAID is set on the 8920 to allow caching frequently used files for any storage in the M.2 slot, such as a 16GB Optane device (optional).  RAID allows the Optane drive (or any other type of SSD used for caching OS and program files for fast startups) to associate with the system drive on which the OS is installed.  Don't turn off RAID just because you are not using one of the better known RAID HDD pairings.  It's more than that.

You can change the default boot OS in Windows 10.  The Windows Club has posted a good tutorial on this at:

tutorial

Leave the RAID setting alone.  It is also used to enable caching OS files so that you can startup much faster.  RAID pairs any caching SSD in the M.2 slot with your HDD so that frequently used files are accessed from the SSD instead of the much slower HDD.