Subj : Re: Can an RPi 4 or 5 power three HDDs?
To   : David Higton
From : druck
Date : Sun Aug 25 2024 23:15:09

On 23/08/2024 20:55, David Higton wrote:
> At the moment I'm running OpenMediaVault on a RasPi 3B+ with two 2.5"
> portable USB HDDs and a 5V 8 amp PSU.  I've wired across the USB
> current detectors and put 9000 microfarads across the 5V supply inside
> the case to support the current surges.  This just about holds up,
> but when the time comes to copy data across from an old drive to a
> third drive prior to replacing the old one, somehow there isn't enough
> power. The numbers say it ought to work, but it doesn't.  So I'm
> considering upgrading to a later Pi model and PSU.

Do you need both of those active when you are copying to a new one,
can't you do it one at a time? Buying a new Pi for temporarily having 3
drives connected doesn't seem to make much sense.

Use a suitably rated powered USB 3 hub for when you are doing the copying.

> I have a RasPi 4 that I haven't used yet.  I see there is an official
> 5.1V 3A PSU.  The extra 100mV should help cover for the inevitable
> ohmic losses, and 3 amps means there might be enough current.
>
> Alternatively I could buy a RasPi 5 and a 5V 5A PSU.  Not keen on
> only 5V, because 5V in meeans a bit less than 5V out to the HDDs.

The power supply for the Pi's are rated to be able to cope with the
maximum rated load of the board and all of the 4 USB ports, and the PCIe
interface on the Pi 5. It doesn't mean you can use devices which exceed
the power output of individual ports, that could damage the Pi.

> I'm looking for the experience of anyone who's actually tried any of
> this and found it to work or not to work.  Please only respond if
> you've been there.

I've used two 2.5" spinning drives, and one HD and one SSD, but I've
never tried 3 as no Pi has more than 2 USB 3 ports, so performance would
be poor. For anything more than 2 I've used a hub both for power and to
maintain USB 3 speeds.

---druck

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