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Issue:

I try to use two external USB 3 2.5 inch hard drives (same make/model) attached to an active USB 3 hub with 4 ports. It is a single board computer with only one USB port. One drive gets recognized, the other spins up and is then somehow put to sleep. Dmesg/lsusb only shows one connected drive. Both drives are functional when connected as only device on the USB hub. Both drives work on another PC.

Questions:

  • Is there a limit on the number of hard drives that may be attached to a single port? The USB hub features 900mA per port.

  • How do I make it work?

migrated from serverfault.com Jan 15 '17 at 3:21

This question came from our site for system and network administrators.

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    Check the power consumption of the HDDs. Sounds like the USB hub is providing not enough power and you might need more hubs. – Thomas Jan 2 '17 at 18:23
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    Thanks for the quick answer. I was on the impression, that an active USB hub should provide the same power to all ports. But I googled some more and found that there are usb y cables for attaching the power of a second port. I will try that. – Reiner Rottmann Jan 2 '17 at 18:48
  • "a single board computer with only one USB port" What kind of SBC? How it is powered? Which OS it is running? – Ale..chenski Jan 15 '17 at 18:12
  • Do I understand this correctly, that ANY of the two drives, when attached alone, work fine, but if the second one is connected, it does not work? – Ale..chenski Jan 15 '17 at 18:18
  • That is correct. I solved it, however with a Y-cable that allows to connect to two usb ports - one for data. the other for additional power. When the second hard drive gets another usb port to draw it's power, both drives work at the same time.This is really strange. – Reiner Rottmann Jan 16 '17 at 17:43
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Answer:

There is no limitation to the hard drives to a single port. The bigger the size the bigger the power use. Please connect hard drive to usb with 2 cord wire. One for power and the other one for data. You cant use your hard drive if just connect a single cable. It must be 2 cable connection. If that port is USB 2.0, just connect to USB 2.0. If 3.0, connect 3.0 or 3.1 connect to 3.1. Connect to the same version of usb.

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The solution to my issue was an USB Y cable that I attached to one of the drives. So one drive got power from two ports of the active USB hub.

According to the USB specification, this is nonsense, as each port should have the same power rating. But it seems that with one drive attached, the power is not enough at each port for two 2.5 inch drives.

USB-Y-Cable

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