Re: that spot on the board is indeed populated with a cable that runs off into the chassis

the chassis is very crowded and the sata power connector may be hidden behind the gpu end bracket. It ought to look like this cable next to the sata data cable

Tried that without much love...but knowledge is power. Do you (or anyone else reading) know if that cable IS supposed to run through the chassis? I had assumed that it was supposed to track around the perimeter of the board and reemerge near the drive tray--but am I wrong?  Is it supposed to run directly from the board to the drive without running through the case? This is the only one of these builds that I've seen, so I'm not sure! I'd rather not have to sit and try to pause during tear down videos to get a glimpse!

@redxps630​ Thank you, you're helping me get closer. If you have one of these beasts, would you mind just seeing where that cable tracks back to? Is it right off the board, or does it come out of the case near the graphics card there? I used to build my own, but went to Dells because I didn't want to take time to troubleshoot on my own, which is ironic here, since now I have to...and since I didn't build it, I don't know where that cable is supposed to come from! I shouldn't have trouble recognizing the SATA power cable when I see it, but it IS possible it's hiding under something and I'm not seeing it. 

I do not have one but R16 and R15 are basically same interior just the plastic shell different.  in this R15 tear down video the sata power cable is tucked right above the 3.5 bay by two cable hooks https://www.youtube.com/watch?v=L1v3B1188vc

playtime 37:29 and 1:27:10

I can't thank you enough, that's EXACTLY what I needed. I'm just going to have Dell come out and "fix" it for me, that cable is nowhere to be found in the area of the drive bay, and it's certainly not primed and ready to go as the data cable was (and as the cables in the video are). I expect this will be a struggle to get them, but good times. 

Once again, thanks for finding that for me, I greatly appreciate it. 

Once again, I appreciate your help. Per Tech Support: because my machine didn't ship with a 3.5" drive, I need to buy part numbers 3W6TM and 1JFJ8 from sales. I found 'em on E-bay, but I'm going to talk to my small business sales rep to see what the heck they are. I'm a little confused because there IS a cable running off of the board at the SATA Power port...but as will often happen I'm going to go through my small business rep because tech support...well,  doesn't offer all that much support.

Once again, thanks for the help!

@albertkaholic​ Ah yes, the design decision to sell you a system with a power supply, but it only comes with the cables that are needed to power up your configuration.

Anything else you need to purchase the correct modular cable, which will likely come at a certain price and might not even be readily available.

God forbid they would put the unused cables in a plastic bag and ship it with your most amazing ever gaming PC.

Do keep us posted on your journey to the SATA power cable.

@Vanadiel​ , yeah, well, they don't have too much love in my heart right now. So am I safe in assuming that the overpriced things they sell me will plug into what appear to be extra graphics cards cables hanging off of the PS, and not the connector on the mobo they label "SATA Power"?