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Pi NAS Power Supply Gotcha

The Problem

I had a Raspberry Pi 5 with a Radxa Penta SATA HAT and four Kingston 894GB SSDs. I assumed the official Raspberry Pi 27W USB-C power supply would be enough to power everything.

It wasn't. The SSDs didn't show up. lsblk showed nothing. lsusb showed nothing. lspci showed the Pi's built-in Broadcom bridge and Ethernet but no SATA controller.

The Investigation

At first I thought it was a driver issue — maybe OMV7 didn't support the HAT. Then I thought the PCIe ribbon cable was bad. I reseated it multiple times, tried a replacement cable, and even double-checked the contact orientation (contacts face down on both ends).

The breakthrough came from checking dmesg | grep -i pcie — no errors, but also no SATA controller detected at all. The HAT simply wasn't getting enough power to initialise.

The Root Cause

The Radxa Penta SATA HAT connects to the Pi 5 via PCIe (FFC ribbon cable), not USB. It uses a JMB585 PCIe-to-SATA bridge chip that needs 12V power to operate.

The official Pi 5 USB-C PSU outputs:

  • 5.1V @ 5.0A
  • 9.0V @ 3.0A
  • 12.0V @ 2.25A (27W)

Even at 12V, it only provides 2.25A (27W). Four SSDs plus the JMB585 controller need approximately 36-48W for stable operation with headroom for write spikes.

More importantly, the connector is wrong — the HAT expects a 5.5×2.5mm barrel jack (center-positive), not USB-C.

How the HAT Actually Powers Everything

This is the non-obvious part. When you plug 12V into the HAT's barrel jack:

  1. The HAT uses 12V directly for the SSDs and SATA controller
  2. The HAT converts 12V down to 5V and back-feeds it to the Pi through the GPIO pins
  3. The Pi boots and operates entirely from power supplied by the HAT

So you end up with one power supply powering everything — but it's a 12V barrel jack adapter to the HAT, not the USB-C supply to the Pi. The Pi's USB-C port goes unused.

The Fix

Bought a universal 12V 5A (60W) DC adapter with interchangeable tips — around £17. Use the 5.5mm outer / 2.5mm inner barrel tip, confirm center-positive polarity.

Minimum spec: 12V 4A (48W). I went with 5A for headroom.

Additional PCIe Config Required

Even with correct power, the Pi 5 needs PCIe explicitly enabled. Add to /boot/firmware/config.txt:

dtparam=pciex1
dtparam=pciex1_gen=3
usb_max_current_enable=1

The pciex1_gen=3 enables Gen 3 speeds. The Pi 5 is only certified for Gen 2 but Gen 3 works reliably for most users. If you get stability issues, drop to pciex1_gen=2.

What I'd Do Differently

Read the HAT's power requirements before ordering a power supply. The Pi ecosystem makes you assume everything runs off USB-C, but any HAT with its own power connector is telling you something important.

Also: don't use both power sources simultaneously. Running the 12V barrel jack and the Pi's USB-C at the same time can cause ground loop issues. Pick one. The HAT's barrel jack is the right one.