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How can I be sure a self-powered usb hub will be able to power several HDD?

What do I have to be aware of?

I'm looking at powering 4+ external HDDs from different vendors and capacities (500GB, 1TB, 2TB) to use with a Raspberry Pi 3 Model B in an amateur home NAS with Open Media Vault software.

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You need to provide enough current to power all the devices. The USB port provides 5 volts. So assuming a realistic worst case scenario you want 2 amps or 10 watts per device - so for 4 devices you need a power supply for the hub which can output 8 amps @ 5 volts or 40 watts.

Your actual requirement will probably be something less depending on your hard drives. 2.5 inch 5400 rpm drives will typically use around 1amp with 7200 rpm drives using more - and 3.5 inch drives will use very little power from the hub as they draw too much power to be powered off USB.

You mentioned a RPI3B - be aware that your speeds will suck. While I'm sure it can be done in theory, I can't see this setup running RAID5 or RAID6 with meaningful speed, but then again, with its 100Base Ethernet port its not going to give you more then 12 megabytes per sec best case scenario anyway (less then 1/10th the speed of a typical hard drive).

(What I'm trying to say, politely, is that this may be doable in theory but will be frustratingly slow in practice. There are most likely better solutions which suck less for not a lot more money. I've not researched much but even a Pine64 will give you a gigabit network card and USB3 interface which at least gives you a shot at usable speeds )

  • Thank you @davidgo did you really mean 12MB/hour ? Would that apply to a simple file transfer over wifi to local network with the rpi on ethernet? Thanks – Jaume Mal Aug 15 '18 at 9:29
  • Good catch. 12 megabytes per sec. That's actually very slow. – davidgo Aug 15 '18 at 9:32
  • I may try with the pi as proof of concept then go for the pine or a rock. When you say I need a power supply that can provide 8A @ 5V or 40W. do you speak about the hub's power supply or the hub itself ? or both ? I am typically seeing 5V@4A. Alternatively perhaps I could do 2xUSB HUB , 5V@4A each. Do you think I could also power the rpi itself from one of those? thanks – Jaume Mal Aug 15 '18 at 14:48
  • The hub itself probably uses very little power, so from a practical POV you can ignore the hubs power draw. Using 2x 4 amp hubs is better then 1x4 (from a data hanging POV). I don't know if you can power your RPI from a USB port, but if you can you need an extra .,1 amp for it. I understand that powering the ROI from the hub - where it works - is not a best practice as a power surge from another device can kill the Pi (and I've killed USB ports with external hdd's before, so it's not just theoretical) See raspberrypi.org/documentation/hardware/raspberrypi/power/… – davidgo Aug 15 '18 at 19:13

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