Dell, in all their penny pinching proprietary wisdom, has made it virtually impossible to mount a 2.5" SSD as a second drive in an OptiPlex 3020. You can get a 2.5" to 3.5" mounting kit but the problem with these kits is that they center the 2.5" drive in the bracket. At that point the power and data connectors aren't exactly where they are in a true 3.5" drive. They are off by about half an inch and Dell, in all their infinite wisdom, doesn't give you a half inch of play in the power supply leads.
The long an short of it is that the 2.5" to 3.5" mounting kit has to leave the power connector and data connector exactly where it is on a true 3.5" drive. Doesn't anyone know of such a mounting kit?