Hi Jan, Here is the manual for your Opti 780: [URL]  There are two slots for a 3.5" HDD so plenty of room inside. You'll most likely need a data cable, but there should be a spare power connector. There are both SATA 1 and SATA 2 ports. Check the manual to see which is which. For either eSATA or USB, you'll need an external enclosure. But any up to date USB 3.0 expansion card should be compatible with your system.
I have a new (refurbished) Optiplex 780 DT desktop computer.  It has eight USB 2.0 connectors, and no USB 3.x connectors. I also have a 1TB 3.5" SATA drive from an old (RIP) machine (WD1001FALS). 1.  What would I need to add a USB 3.0 upgrade card so I could use the external USB-SATA interface that I have at USB 3.x speed.  I know about the board fitting into the case, but what power and functional interface connections are available  inside the case, or at least on the main board?  2.  What if I wanted to make this an internal SATA drive, are there a connectors (power and interface) inside for that?  There is a front panel blank that implies that I could fit the drive in there, possibly with an aftermarket bracket.  Alternatively, I have positioned this as a mini-tower, so I could simply lay the drive inside wherever it fits. 3.  If I were to choose to use the esata connector on the back, is there a way to get power to the drive outside the box? Thank you, Jan  
Add SATA 3.5" drive to Optiplex 780