I bought a HP ProLiant DL180 Gen9 server in 12 LFF (3.5-inch) HDDs configuration. Now I'd like to put two additional 2.5-inch SATA HDDs inside the case to serve as system drives.
As a configuration with 12 LFF HDDs, this server has a P840 PCIe card that provides SAS/SATA connectivity for the whole HDD backplane. An integrated B140i SATA controller is to be used for those two additional 2.5-inch HDDs.
The remaining part of the puzzle is to somehow provide power to those two additional HDDs.
Some DL180 models can be stuffed with an optical drive (though this one isn't, but the system boards are/should be the same). Documentation shows that the optical drive power and the hard drives power come out from the same port (go back 3-4 pages). The port and the connector are here
Judging by the official HP documentation and by having five unpopulated pins, I'd say that those exact five pins provide power to the optional optical disk drive.
My actual question is what's the function of unpopulated pins in the 20-pin connector? I need to find +12V, +5V and two GNDs. A friend of mine and I have disassembled the whole casing, took the system board out in hope of finding the power lines on the bottom side of the board, but it looks like they are in some middle layer. The last resort is to find pin-out by using a voltmeter, but I don't know if I could make something let out the blue smoke :)
Thanks in advance,