For anybody interested I share my idea on how to power R2 including the attached HDDs. As I need to run R2 with two HDDs (one of 2.5" and second of 3.5" size) I had to sort out the problem of how to power it all optimally. After some googling I found power brick that provides both 12V and 5V outputs via standard molex connector. And that did the trick for me.
As you can see from the picture below I power the 2.5" disk from R2 but the 3.5" one directly from power adaptor via molex as I’m not sure if R2 can provide enough current to spin it up (didn’t try it though). I also brought 5V from adaptor to microUSB port on R2 to make it boot automatically after power lost.
Here is the detail of the power brick. It is rated 2A for both 5V and 12V output lines. Unfortunately not clear if meant simultaneously, ie. 4A in total or not. But it works well in my setup. I have attached a dual DVB-T USB donge to R2 too.