I was not able to power an external HD directly from a Raspberry pi 3 (the HD was constantly complaining about a lack of power). Would a Raspberry Pi 4 be able to power a portable hard drive (using the standard charger)?

  • What type of HD? A 2.5" hard drive, a SSD, a 3.5" drive? – Fred Sep 20 '19 at 22:20
  • I have no trouble powering a 2.5" HDD on the pi 4 if that helps – Jaromanda X Sep 21 '19 at 4:34
  • I have checked that Seagate 2TB takes about 400mA. So if you are using 5V 3A for Rpi4, you more or less make it. But I would recommend to use SSD, whose price is falling rapidly: raspberrypi.stackexchange.com/questions/103784/… – tlfong01 Sep 27 '19 at 13:49

not able to power ... from a Raspberry pi 3

Here's why: Pi 3 was designed with USB 2.0 specs, including the 0.5 amp limit. Most hard drives require more current than that.

Raspberry Pi 4 has USB 3.0, which means it is designed to supply up to 1 amp. That is likely enough to run the hard drive.

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    This answer assumes "standard charger" means the official 3A power supply. – Botspot Sep 20 '19 at 20:17


hdd/ssd power requirements If your Rpi4B's power supply meets the official spec of 3A output, then it is OK to use USB HDS/SSD without external power supplies.

I measured the idle current and write current of a 1TB SSD, and a2TB HDD.

SSD's idle current is 0mA, and write current around 180mA

HDD's idle (spinning) current is 170mA, and write current 200mA ~ 380mA

My conclusion is that for hobbyist's python programming projects with casual use of SSD or HDD, there is no need to supply external power to the USB hub.

If two SSDs are used for professional projects, it is better to supply external power to the USB hub.


(1) USB HDD/SSD Power Requirements

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    I suggest you include the initial spin-up current, as that would seem to be the highest. – Botspot Sep 22 '19 at 12:23
  • @Botspot, Good idea, let me search my junk posts. – tlfong01 Sep 22 '19 at 13:02
  • @Botspot, Just now I searched my old posts on a couple of forums I have been hopping around, and found that I usually started discussions with a colourful picture, and I think I shall use this picture to start another discussion: penzu.com/public/a25b1076. I need to stare at this picture for 3 minutes, to refresh my memory! :) – tlfong01 Sep 22 '19 at 13:18
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    Yes, when a motor is initially spinning up, it takes more current than when it is coasting at full speed. (This is because it has to combat inertia + friction instead of only friction). – Botspot Sep 22 '19 at 18:59
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    That's why large drill press machines have a two-stage startup switch: the first setting on the switch limits the current. Once the motor reaches full speed, the operator clicks the switch again. If they didn't have this feature, the drill presses would likely trip a breaker! – Botspot Sep 22 '19 at 19:01

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