stigzler
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Pi3 Running an external USB3 Hard Drive

Thu Jul 06, 2017 10:23 pm

My apologies - sure this has been asked a million times, but now googling round in circles!

My question is this: Can I run a USB3 HD purely off the RPi power alone?

I'm going to be using a switching 5V 3A PSU (http://cpc.farnell.com/ideal-power/25hk ... dp/PW03339 - not the official psu for design purposes - panel mounting etc). Will also be using the OnOffShim between the power and the board (https://shop.pimoroni.com/collections/r ... onoff-shim).

There will be few other usb things used. Possibly a bluetooth for PS3 controllers. And sometimes a dongle for wireless keyboard (like http://www.ebay.co.uk/itm/2-4GHz-Fly-Ai ... SwZQxW4S3L).

May also be using Wireless.

I've seen various answers on this, which of course range from "no!" to "yes!"

I'd prefer not to have to use a powered usb hub if possible, again due to design considerations.

Informed advice appreciated!

W. H. Heydt
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Re: Pi3 Running an external USB3 Hard Drive

Thu Jul 06, 2017 11:10 pm

It depends on the HDD. The WD PiDrives (they expose a USB3 interface) are specifically designed to minimize peak current load. As such, they can be run directly attached to a USB port on a Pi2 or Pi3. Other drives, probably not, as they won't have that particular design feature and may try to draw more current than a Pi can provide at times.

Also bear in mind that you may not get the performance your are expecting as the Pis only have USB2, which caps transfer rates at about 35MB/s. A test I did with an SBC that has a native USB3 had a rate of 80MB/s with a 314GB PiDrive and about 125MB/s with a SATA III SSD, using a USB3 to SATA adapter cable.

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davidcoton
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Re: Pi3 Running an external USB3 Hard Drive

Thu Jul 06, 2017 11:17 pm

If your PSU maintains 5V at full 3A load, that should be enough for most operations. The Pi is not likely to get low voltage, unless the wiring from supply to Pi is too long and/or too thin (use at least 18SWG/AWG).

The difficulty is with the spin-up current of the HD -- if this exceeds the 1.2A allowed for the Pi's USB ports, it may fail to spin up. It may also drag the USB voltage down, which may affect other USB peripherals, which may in turn affect the Pi.

There are too many unknowns to give a definitive answer, you will have to try it and see. Bear in mind, though, that while it may seem OK on test, you have no easy way to tell how much you have in reserve. On a different day, under even slightly different conditions, there may be a problem.

If you want a conservative (low risk) design, run separate supply wires from the PSU (or switch output) and the HD.

tl;dr? We just don't know. Suck it and see.

Edit: typo
Last edited by davidcoton on Fri Jul 07, 2017 12:14 am, edited 1 time in total.
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peterlite
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Re: Pi3 Running an external USB3 Hard Drive

Thu Jul 06, 2017 11:49 pm

No problems with dozens of 2.5" disks. They are usually 5900 RPM, not 7200 RPM. 0.8 amps is a common peak startup. You can run one but not two. Plenty of power left for keyboard, mouse, extra Wifi adapter. There are lots of pasts on the same topic under NAS. Note that some USB3-SATA chips are really USB 2.5 and will create intermittent problems related to the chip. Run lsusb to get the USB chip id then google the chip id.

W. H. Heydt
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Re: Pi3 Running an external USB3 Hard Drive

Fri Jul 07, 2017 1:49 am

peterlite wrote:No problems with dozens of 2.5" disks. They are usually 5900 RPM, not 7200 RPM. 0.8 amps is a common peak startup. You can run one but not two. Plenty of power left for keyboard, mouse, extra Wifi adapter. There are lots of pasts on the same topic under NAS. Note that some USB3-SATA chips are really USB 2.5 and will create intermittent problems related to the chip. Run lsusb to get the USB chip id then google the chip id.
/nitpick on
5400 rpm
/nitpick off

Then there are (were?) the WD Raptor drives...10Krpm.

stigzler
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Re: Pi3 Running an external USB3 Hard Drive

Fri Jul 07, 2017 7:18 am

Heh. Thanks folks. So no, maybe, yes.. lol
Its looking on balance that it should be ok pending which drive I use.
Peterlite, you mention youve used dozens of drives? Given your warning, which drives work?

vichy8
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Re: Pi3 Running an external USB3 Hard Drive

Sun Jul 16, 2017 4:37 am

It depend on the situation.

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HawaiianPi
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Re: Pi3 Running an external USB3 Hard Drive

Mon Jul 17, 2017 9:27 am

If it is a desktop (3.5") based drive with its own power supply, then definitely yes.

If it is a portable, USB powered drive, then very likely, but it will depend on what else you have the USB ports powering, and how voltage stable your PSU is.

The USB 2.0 standard is 500mA per port, so USB 2.0 hard drives are designed to run within those power limitations. Also note that a computer with 4 USB 2.0 ports should output a total current of 2A for all 4 ports combined. This is why some USB hard drives come with a Y-cable that connects to 2 USB ports. The extra plug gives an extra 500mA for drives can't quite meet the 500mA USB 2.0 power limitation.

The Pi3 can output a max of 1.2A for all 4 ports, which is less than the USB 2.0 standard. However, the Pi3 has no per-port limits, so it can output 1.2A from a single port and should be able to run a USB 3.0 HDD, as long as you aren't using other power hungry USB devices (if the total current draw from the USB ports exceeds 1.2A then something's gonna fail).

Another problem is voltage droop. Many 5V power supplies are designed for charging phone or tablet batteries, and batteries are not very sensitive to voltage fluctuations. As a result, many so called "5V" power supplies output much less than 5V under load (4.5V will charge a phone battery, but it won't ruin a Pi3).

Unfortunately there are too many variables involved to give you a definitive answer.

All that being said, I have used a variety of USB 2.0 and USB 3.0 portable hard drives on my Pi3 without any problems. Note, however, that my Pi3 is powered by a 5.3V/2.4A PSU with pretty good regulation. I have even successfully spun up 2 USB powered portable hard drives, but I could only access 1 at a time.
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Ernst
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Re: Pi3 Running an external USB3 Hard Drive

Mon Jul 17, 2017 12:18 pm

Something that has not yet been mentioned: for best results with USB2 attached hard disks it is important to use high quality (~ thick) and short USB cables to keep the voltage drop to a minimum, where by short means ~ 10cm..15cm (4"..6").
My experience external hard disks using USB3 is that the length of the USB3 cable (came with the disk) can be a bit longer even when plugged into a USB2 socket.
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peterlite
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Re: Pi3 Running an external USB3 Hard Drive

Fri Jul 21, 2017 1:40 am

stigzler wrote:Heh. Thanks folks. So no, maybe, yes.. lol
Its looking on balance that it should be ok pending which drive I use.
Peterlite, you mention youve used dozens of drives? Given your warning, which drives work?
For USB chip problems, look at posts like the following:
https://www.raspberrypi.org/forums/view ... 3&t=182942
https://www.raspberrypi.org/forums/view ... 2&t=178162
https://www.raspberrypi.org/forums/view ... 3&t=174619

Run lsusb and google the chip USB ID.

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