I have recently purchased an OpenRD-Ultimate from NewIT and I can confirm that it's possible to install a 2.5" SATA hard drive internally. No SATA cable is required as the disk directly fits in the connector. You just have to buy a 2.5" SATA disk and screws.
But without the Spacers you could damage your OpenRD, because the HDD could get connected to the PCB and short the circuit.
And the connector (without the spacers is a bit build to high) so that the HDD could be installed in a even order...
You also need some longer screws than for a normal 2.5" HDD.
So installing a HDD to a OpenRD out of the box is impossible :-(