5040 SFF would use tiny sata 2.5 drives only due to power constraints. And the drive would have separate data and power cable. Not enough power for 15W 3.5 DRIVE OR SPACE the blue caddy is  [URL]   [URL] [URL] [URL] [URL]   [IMAGE]hard drive  
I have purchased a used Optiplex 5040 SFF that has only a 128Gb SSD drive. I want to add a 3.5" HDD. The drive tray does not have a space to add a 3.5" HDD. How can I find the correct tray that will let me keep the SSD drive and add the HDD drive? It appears that I will need to add a SATA power splitter. Other than that I think this will work.
How to select a Optiplex 5040 drive tray