[NAME], The numbers only relate to the device connection number and how the system and OS sees them - to know which drive is which and where it's connected. Nothing to do with speed.  I ordered a 3891 for my son last Christmas with a 256 GB SSD and an optical drive. I added a 1 TB 3.5" hard disc drive to it the day I got it. I used the connectors provided and had no problems. There was an extra SATA cable already plugged into SATA-0, and an extra power connector. The thing works just fine, and I'm sure a hard drive pulls more juice than an SSD drive.  You shouldn't have any problems. Just make sure you go into the Disk Management applet in Windows to activate and tell the PC which drive letter to give it after you install it, because Windows won't see it at first.  The system BIOS will, but Windows won't until you activate it.
  Inspiron 3891 came with one HD, the 512GB  M.2 NVMe, with Windows 10 pro installed.  I want to add more storage since it looks tricky to partition the SSD to create a D drive.   The motherboard has an unused SATA slot (SATA1), and an empty 3.5" bay in the chassis front but no extra cabling for it.  But, the 3891 specs are ambiguous (to me) as to ability to support a new SATA drive (plus the optical drive on SATA3 already).  It's not among the 3 drive combinations listed as choices for the model--unless Dell just doesn't sell the 3891 that way (with ODD plus 512 SSD , and a SATA HD).  A real operational problem or not?  Some posts and youtube suggest such options (for 3880).  Clear advice would be appreciated, been stymied for weeks now.  --D
3891 add a SATA HD?