I got the same issue. My OS 2TB SSD became Disk 1 and 4TB for media files is currently Disk 0. Doesn't effect the system booting or performance, but just annoying. [IMAGE]  
I have an XPS 17 (9700) which I bought a few months ago with a single drive. Last week, I bought a 2nd drive (also from Dell) which was installed by a Dell Partner technician. For some reason, the technician first removed the original drive and, unfortunately, I wasn’t paying enough attention to know whether that drive was in slot 1 or slot 2. So he did what would be pretty standard, or at least was so with older 17” Dell laptops and put the old drive in slot 1, being the drive with the OS installed on it, and put the new drive in slot 2. He then successfully ran all the diagnostics. The XPS booted fine but the new drive became the new Disk 0 while the old OS C-Drive became the new Disk 1. I would like to correct this before going any further. How should I go about it?    Other question: what should the disk configuration be, RAID, AHCI or Disabled and why?   I appreciate  any help you can give me.    Thank you Waguih B  
2nd Drive on XPS 17 (9700)