Short answer: It works in principle, but the hub may not be able to supply enough power for the entire setup. Not recommended for production use.
I have taken the risk and in the meantime set up two Pis as described above:
+------------------------------+
| USB hub |
+--(uplink)---(port)---(port)--+
| | |
| | |
| | |
+--(USB)----(power)--+ +----+
| Pi | | HD |
+--------------------+ +----+
- Pi 1: first generation, with a USB hub purchased recently (2022).
- Pi 2: 3rd generation, with a USB hub purchased prior to 2011.
Both hubs are USB 2.0, with a power supply rated for 2.5A, which looks sufficient to me (the original Pi adapter is rated at 5.1V, 2.5A).
Initially everything worked, nothing got fried. Both Pis booted up and could access the hard disk. So the Pi does not seem to have any issues with a double connection to the hub per se.
After running this setup for a few months, however, power seems to be an issue. On both Pis I keep seeing the thunderbolt on the local screen, and get frequent under-voltage warnings. Every once in a while, the Pi stops responding, or loses its hard disk. Sometimes the disk does not spin up when I power on the system. All of this indicates power issues.
I tried replacing the USB cables involved (from the hub to the Pi and to the disk) with Y cables, which have a second USB A plug for extra power – to no avail, as the Pi still reports power issues. That indicates I have hit the limit of the hub as a whole, not of a single port. (These tests were only carried out on the Pi 3 so far.) If I power the Pi via the official power supply (which I have tested only briefly), power seems to be OK.
After a recent outage ended up corrupting the data on my disk, requiring a lengthy recovery operation, I started looking into alternatives.
Alternatives (if the hub setup is not stable enough)
Of course, you can power the Pi with its original power supply (or one specifically manufactured for the Pi).
Apart from that, Geekworm makes an enclosure, extension board and power supply tailored to the NAS setup I have in mind. I would get a (hopefully) stable power supply along with an enclosure that looks more professional than a Pi, disk and hub taped together. This allows me to keep the Pi 3 with its system setup, though I will need to buy the whole kit (about € 100) along with a new hard disk.*
*If I am lucky, the external HD is a SATA disk with an external USB adapter, all mounted into an enclosure, so I can just rip out the disk and mount it into a SATA enclosure. However, many of these disks have a “native” USB interface and thus cannot be used as SATA disks. Seagate/Maxtor disks are typically modular (though removing the disk from the enclosure and using it as a SATA disk voids the warranty) while Western Digital uses native USB disks which cannot be repurposed in this way.
You would need:
- The X735 power & fan board, ca. € 30
- The X820 SATA extension board, ca. € 40
- The X820 Metal Case, ca. € 30
- A 2.5" SATA disk of the desired size
- A power supply (with the X735 V3.0, anything in the 6–30V range and capable of supplying ~20VA is OK – I had a bunch of suitable ones still lying around)
- A Raspberry Pi 3 (Pi 2 might also work; the Pi 1 or 4 are mechanically incompatible with the enclosure) and SD card
There is also the X835 board, designed for the Pi 4 and 3.5" SATA disks, and the matching enclosure. The X735 is compatible with both.
I have tested this setup for a couple of weeks now, and so far things have been stable enough to convince me to acquire another kit for the second Pi. Two caveats, though:
- If you want to use the full potential of the power board (i.e. fan turning off and power LED going dark on shutdown), you will need to install some extra software provided by Geekworm. Also, you need to use their script to shut down the Pi. Without that, the fan will keep running and the power LED will remain on after you shut down the Pi. OTOH, this is not really an issue for a system that is running permanently anyway, and still a lot better than the USB hub solution.
- The case can only take disks up to 9 mm in height, which kind of defeats the purpose of a NAS (as I was unable to find a 4 TB disk less than 15 mm in height). I was able to overcome this limitation by cutting the case in half and inserting a metal strip, stretching the case by 5 mm, and installing larger spacers on the HDD adapter board – this works but does not look quite as professional. Of course, you can also run the whole thing without an enclosure.
