My machine is a 790-0031DT, it has the same power supply and chassis as the 790-020 and similar motherboard.
The SATA Power connections in this machine are not the standard. It doesn't come with power line connectors for added drives, or even the normal 4-pin MOLEX connectors that most SATA power cables are made to tie in with. You're right that SATA connectors for power are common and easy to find, but all the ones I have are set up for 4-pin MOLEX power connections.
Instead there's a six-pin mini ATX female connector on the motherboard labeled "SATA power" (red circle in photo). There's also a free 6/8 pin male ATX connector harness on the power supply--I believe it's designed for graphics cards that need their own power supply (blue circle). 
The problem I'm having is finding a cable which has SATA 15-pin power connectors on one end and either of these connectors on the other. So far I bought a cable but it had full-size six pin ATX instead of mini.
Not to mention that HP doesn't have this info anywhere, or a way to buy such a cable on their site.