TL;DR - it's the MiniSAS
First things I tried was basic troubleshooting - different VGA cable, different power cable. No luck there so I tried with the original RAM - still no progress.
I logged a job with HP who diagnosed a motherboard fault and sent out a replacement board. After swapping the boards over I started the machine up with video and a blue (good) status light but realised the MiniSAS cable wasn't plugged in and once I did the machine refused to boot.
I was sent a new MiniSAS cable and managed to replace it - not an easy job. The server booted and is now working as expected.
If you have this issue, try unplugging the MiniSAS and see if the system boots. If it doesn't, contact HP or find a supplier for a HP Part Number: 616572-001 or HP Spare
number 624881-001. The cable looks like this:
I had difficulty finding instructions to replace the cable so here is how I did it. HP also provide an instruction video here.
To remove the MiniSAS cable you will need to remove the MiniSAS from the motherboard (can be difficult) and for ease of access remove the motherboard and tray, and the 4x Hard disk caddies. You will then need to remove 4 screws at the back of the chassis. The allen key that comes with the machine and should be stored in the door can be used for this. The locations are marked below.
Once the four screws are removed the MiniSAS cable needs to be fed back though the chassis, over the power supply, but you will need to cut a number of cable ties to do this. Then you will need to disconnect the 4 Molex connectors (if you have a hard disk or CD/DVD drive in the top bay this will need to be removed to gain access to the cables). This was not easy as 1 of the cables was very stiff and needed some coaxing to come out.
Once the screws and cables are loose the rear panel that was unscrewed can be removed - I had to bring it forward feeding the cables down behind it and then rotate it so it could slip out between a center stand in the disk caddy area. You then end up with this (actually upside-down here).
The cable can then be disconnected from the mounting plate - 8 screws can be removed using the tool that comes with the server. The replacement cable can be pushed into place (small plastic bumps line up with holes in the rear and then screwed in. The plate can be put back into place, feeding the cables up past the fan, and then screwing back into place. Molex cables need to be connected back up, the MiniSAS fed back through over the PSU. Motherboard can be put back into place and MiniSAS, Power and other connectors can be done as well.
Put the door back on, cover on, power in and VGA in and start it up. That should be it.
Good luck.
Yes, this is where SAS expanders, enclosure backplanes and breakout cables come in. You can obtain a breakout cable that will provide 4 ports from the SFF-8087 mini-SAS connector. You can also try to use something like the HP SAS Expander to provide more SFF-8087 ports for your use from the single RAID controller. A detailed forum about the HP expander is available here.
I mentioned backplanes only because some have embedded expanders that allow them to accommodate more than 4 or 8 drives. You're probably not working with a system with a SAS backplane, so disregard that part and focus on expansion at the RAID card level.
Best Answer
I have a number of these servers and it is awkward to disconnect, although it does sound odd you are having this amount of trouble
Tips, because of how the cable is routed and cable tied, also the length of the connector, you need to pull directly up while putting a kink in the actual cable.
The metal clip seems to depress further if you place your finger in the middle. It does latch in to the socket fairly well so getting it depressed far enough is important.
The machine will post without the SAS cable connected so even if you have managed to pull it out slightly, the blue logo should light and the system post. Things that will stop it from booting are having the fan header disconnected (From memory I think that is the cable closest to the SAS connector)