So I've given up on this build, but it makes no sense to me as to why it isn't working. Aside from assuming it *should* work from many people's similar builds, there is clearly something else that I'm missing here. I am now only really seeking to understand the problem, so it can possibly help me in future builds.

The build:
Gigabyte H61N-USB3 (ITX board)
Core i3 2105 (Needed and only running onboard HD3000 graphics) @ 65W TDP
1x2.5 7200 RPM 2.5" Laptop drive (WD Black)
1x4GB Stick DDR3 1333 (Yes just one stick)

That's it. Now I have a "similar" ITX build running off a 120W PicoPSU+ ONLY an 80W AC Adapter. That build is as follows: H55N-USB3, Core i3 530 (running onboard graphics)@ 73W TDP, 1x2.5" 7200RPM drive, plus 1xWD Green Drive. This build works beautifully, it boots perfectly, it's mostly a file server, hence why I'm ok leaving it with an 80W AC Adapter.

The new H61N-USB3 build isn't working. It won't boot with the PicoPSU120W+120W AC Adapter I bought for it. It attempts to, lights/fans come on for a second, but turns off immediately, and tries to start up again. This loops until I kill the power. It's behaving as if it's not getting enough juice at POST. I've verified all the parts work. Tried the new PicoPSU on my H55N-USB3 server, works. Tried the brick too. I tried my server's PicoPSU and brick on the new build, and it behaves the same way, it won't POST. Why? The new H61N-USB3 build works perfectly, when I use a full sized ATX PSU with it. So it doesn't appear that anything is broken. This combination simply isn't coming together. =/. Oh, and I did try unplugging the drive, and even the fan to try and get it to POST.

I would appreciate some insight here, as in my head this should be working great, but it's not. After all the wasted time on this build, I'm pretty sure I'll be scrapping it.

Sometimes, when i disconnected the brick from the outlet, it takes a couple of unplugging and re plugging, its like something doesn't start well, but once it does never have given me issues until i unplug it again, so my advise is to try unplugging it multiple times from the outlet, even let it sit without plugging, etc, maybe its the same issue as mine.

I'm curious too; that should need half the oomph of 120W brick.

Did you have anything plugged into the PCIe slot while checking the H61N-USB3 with the PicoPSUs? The only POST related issue I found after a quick search was to do with a PCIe Wireless-N card causing the Gigabyte board to behave like you mentioned, but they were using a Corsair AX850 PSU. Also, make sure the BIOS is up to date.

If none of this helps, I suggest checking the desktop PSU again and make certain that absolutely everything connected to the motherboard is the same except the PSU. If it's not able to POST now with the desktop PSU, then try a different mouse or keyboard or unplug the USB header cable if it's connected to the motherboard, etc. Try to get it working again with the desktop PSU.

If the desktop PSU was fine, then there is still something weird going on with the motherboard I think. Maybe check what power management options are enabled in the BIOS?

edit:
Most important question I should have started with was, how many pins do your 120W PicoPSUs have, 20 or 24? Do they have the 12V 4-pin cable or are you using an adapter?

Some boards are fine with 20 pins, others require 24 which can be done using a 20-to-24 pin adapter. Apparently, some also work without a 12V 4-pin cable connected, but I haven't seen one myself.

what brick is it?
sounds like its lacking enough 12v amps to pass through the pico to the chipset.
'prolly need a better and more powerful brick...?

Thanks for the replies =).

No there is nothing plugged into the PCI-E Slot. The specs above are literally it. Sure I had a usb mouse and kb connected, but during my troubleshooting I unplugged everything to try and get it to POST (even CPU fan =P). No go.

Everything is the same when going between PSU's. There nothing I could/would have changed. Yet every time I've gone to the ATX PSU, it worked perfectly. That's actually how I got the OS fully set up, I was awaiting the PicoPSU in the mail, hoping to simply plug it in and have it ready to go. =/.

As far as BIOS options, I didn't think there was much I could change in there, aside from trying to undervolt things.

20 to 24pin adapter! Guess that's something to try, I have one too I think. I'll give it a shot here now. It's a 20Pin PicoPSU, it doesn't have the P4 cable, so I'm using an adapter.

Edit: The AC Adapter isn't top quality, it was bundled with the PicoPSU and an M350 case. I didn't think much about it considering that it worked just fine on my server, which has an extra drive, and an generation older chipset + higher TDP CPU.

Edit: Tried the 20 to 24 pin adapter, it made no difference. Is it really possible it's the AC Adapter not having enough amps? I would think the lower power components wouldn't even need as much as this generic AC Adapter can do.

I'm thinking you didn't use the 12V P4 adapter when testing with the full sized ATX PSU? That's the only variable I could imagine exists now between the power supplies. If you could test that and it makes no difference, then I think the 120W adapter isn't working correctly.

Are you suggesting it's possibly the 12v P4 Adapter that's at fault? If so, I don't think that's the case, as I was troubleshooting with the same one that's in use in my file server. I'm going to try swapping yet another brick, and try the ATX PSU again.

That's right. I was thinking if it was me in your situation, I'd have accidently tested with a different P4 adapter in each computer and I'd realise it several days after I gave up

If true, it would explain why the 120W brick worked in the File Server but not in the new H61N computer.

Well, to add to the mystery, I re-connected the ATX PSU and it won't POST now either lol. Ah. It doesn't immediately shut off like it does with the PicoPSU, the fan stays on, but there is no video output. This, after countless times using the ATX PSU to set it up the OS/apps, and then even going back to it repeatedly, consistently working every time during my troubleshooting of the PicoPSU! A now defective board? I hope I wasn't the cause =P. Oh, and I don't think it's the ATX PSU at fault either, tested it on another build and it posted fine, after a slight hiccup (prob a cold boot issue deal).

I think I'm done with all testing and troubleshooting, going to pack it all up.

Edit: Got it to boot again with the ATX PSU, grr, I don't know what that was about. Shall I dare try the PicoPSU again? =/

Last Edit: Tried a higher quality AC Adapter, though it is only 80W. I noticed it got a little further along in POST. The monitor came on, turned off immediately, but that's one step ahead of the lower quality 120W brick. So again, it seems it's not getting enough juice, but I would think this build would be significantly lower power than my server, and not require anywhere near 120W. =/ This thing is going back.

Have you tested it to see if it might be a bad motherboard or CPU?

This might sound bad but try reinstalling the os. I had a similar problem.

take the battery out {cmos} and see if it posts