This is the cable D/PN 025X39 with a black tab[IMAGE][IMAGE] (the right upper corner sata data cable has a blue tab) it is not included in 8950 chassis by default it is included in XPS 8960 even if no hdd shipped for the left upper corner 3.5” hdd cage.
many XPS 8950 shipped without this Dell oem cable, thus a number of users have asked where to get this cable.  There are a number of solutions including a 75 cm long red cable that has got good review.  SataGear 30in Red SATA Device Cable Straight to Left Angle R-SS-075MSL (and Dell PowerEdge R900 MX550 76.5 cm Long SATA Cable, blue)   Looking at XPS 8950 pic, this cable with a black connector seems to be present in only Dell 8950 promo pic, and absent in almost all retail unit.  What is Dell’s game here sending users everywhere to hunt for this cable? after some investigative work, it seems this cable is similar to the one used in XPS 8940 for the second 2.5 hdd (left upper) with the correct angle bracket.  8940 has sata ports on right side similar to 8950.  the next step is to ID the DP/N which is conveniently missing in 8940 part list.  Based on internet photo and guess work of partially illegible sticker, the DP/N cable is finally identified to be R4X43.  made by Foxconn, available on eBay. NB: this cable is 38.5 cm, 15”.  8950 chassis is 5” longer than 8940.  so this cable might not be long enough in 8950.   [IMAGE] [IMAGE]   [IMAGE]XPS 8940PS here is Dell sata data cable for 3.5 hdd (right upper) with a blue connector.  
XPS 8950 second 3.5 HDD (left upper) Dell sata data cable