optiplex 9020 uses Dell proprietary psu that is prototype of new ATX12VO standard. PSU only supplies 12V to motherboard.  Motherboard generates and supplies additional 5V and 3.3V for sata devices.  Optiplex 9020 MT supports two 3.5/2.5” hdd/ssd.  The sata power cable (HDD and optical drive power connector, #15 below) should have two sata connectors available.   Re: One goes to the motherboard and the other to the hard drive power supply.  If your current cable has only one connector, you need to get a new cable.  Motherboard has only one sata power 6 pin connector socket. The other end of new cable has two sata connectors. You also need a sata data cable if not already provided. [IMAGE]   Need further assistance? Use the "Get Help Now" option at the bottom right to chat with a Dell technician right away.  [IMAGE]      
Hi. I'm new to building and messing around with computers. I looked at the manual but didn't find what I needed. I ordered a 1tb SSD drive and I do see that I there will be two sata cables. One goes to the motherboard and the other to the hard drive power supply. If I ordered another pair of these cables, would I be able to add a regular 1tb HDD (not SSD). Thanks!
Adding a second hard drive to an optiplex 9020