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I have a external hard drive that has written on it "5V ⎓ 1A". Both power and data transfer happen over a single USB. I have been reading about powering this hard drive with the USB ports on my Raspberry Pi 3 B+, but I want to make sure I'm thinking about this correctly. I have a 5V ⎓ 2.5A micro-USB power cord for the Raspberry Pi, the Pi itself, the hard drive, HDMI output to a screen, and a USB receiver for my wireless mouse/keyboard. I calculate it like this:

Total USB current needed (Raspberry Pi 3 B+ can supply 5V ⎓ 1.2A via its USB ports):
1A (external hard drive)
+ <0.05A (my best guess for USB receiver for wireless keyboard + mouse, from here)
= ~1.05A total USB current needed

Then, to make sure the power cord is sufficient:
2.5A (my micro-USB power cord)
- 0.5A (typical current draw of the Raspberry Pi itself)
- ~0.05A (HDMI output)
- 1.05A (USB current needed, shown above)
= ~0.9A leftover.

From my understanding, this seems to be acceptable. I've even tried it a handful of times and was able to access the drive. However, from some sources (see answers in How Can My Raspberry Pi 3 B+ Power All My Devices? and Powering hard drives without a powered USB hub?, and also the Raspberry Pi forum), it seems like there might be surges in the current that the hard drive needs that aren't accounted for on the "5V ⎓ 1A" label. Is this true—will it sometimes require more current than the label suggests? Will I ruin this drive if I don't use a powered USB hub? Thank you for any help you can give!

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You SHOULD be able to power a USB HDD from the Pi3B+ as the USB ports can supply 1.2A.

HOWEVER, I and others have found that this is often unreliable - this seems to be due to transient voltage drop when the disk starts.

I have conducted a number of tests, and often the voltage on the USB ports is low. (I an using a rock solid external 5V power supply with decent home made cables, so the issue is not with the supply TO the Pi.)

The Pi can definitely supply 1.2A to a static load.

SeeRaspberry Pi Power Limitations for discussion.

Ultimately this question is unanswerable, as it depends on the USB HDD.

USB3 HDD often draw up to 900mA and this will almost certainly cause problems with marginal power supplies - indeed almost all power cables use wire that is too thin.

I find older HDD are more likely to be problematic.

If I am doing something important I use a powered hub.

  • Approximately what are the magnitudes of current demand when the disk starts? In other words, what should the powered hub USB ports be rated at so they don't suffer from the same problem as the Pi USB ports? – Eric Mar 12 at 2:57
  • @Eric Unfortunately this is unanswerable. Very few publish meaningful specs; indeed few USB ports (on hubs, the Pi and other computers) are actually standards compliant (Apple seems to be one of the few exceptions). The only way to know is to test the device. Having said that most Brand Name devices from reputable manufacturers will work - avoid cheapies. – Milliways Mar 12 at 3:12

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