As the project continues, things are moving forward. I decided to add a 12 V power supply, which is capable of 3 A output, so everything will be fitted in one box. Since the Iris board and hard drive consume only 316 mA at idle that will be more than enough.
Actually that gives me capability of hooking up different external devices. For that purpose I also designed an expansion board for Iris, which plugs on X16 connector. It has 2 power outputs (relay driven), 4 direct connections to GPIO pins, I2C and UART.
As I said in the previous post the Iris board has only one USB port for hosting other devices, so I ordered a cheap 4 port USB hub for 1.28 $ from eBay. I disassembled the case and mounted the hub on my expansion board. I will use one port for HDD, 2 on the front panel, and one at the back.