HDD wont spin up

Just to clarify, you're not seeing the HDDs in the UEFI, or you're just not hearing them power up when booting?

Did you try a different SATA port on your mobo? Also, I doubt your mobo support hot-plug, so I don't really expect the PC to pick them on the fly.

If you simply can't see them in UEFI, then you might need a newer version or some specific setting? I'm afraid that my knowledge of this board is really limited.

Do you use Molex adapters to power the HDD/DVD or you use the original PSU cabling?

Please post the exact PSU model (I'm guessing CX750M) and some pictures, if possible.

Due to different voltages needed for the different PC components, e.g. 3.3V, 5V, 12V the PSU have different 'power rails' (in most cases). See this example, courtesy of HardwareInsights:



The different rails have different power ratings, all together they are totaling to your 750W. Rest assured, you should be fine with power headroom.

I wonder if there is an issue with the 3.3V or the 5V? Or bad cable or PSU?

I'm starting to notice a trend with these AB350 Pro4.

On a side note, SATA cables that go into the motherboard do NOT provide power to HDD & DVD drives. These drives need additional power that comes from specific connectors from the PSU.

If your Samsung EVO is a M2 drive (like a stick of chewing gum) it suck power directly from the MOBO. If it's SATA (840 EVO for example) then it also needs a separate power connector.

How do you know the SSD works fine? Sounds the PSU can't provide enough 'juice' for the more power-demanding HDD & DVD.