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I have two Raspberry Pi B+. One of the two is rather unstable (details below) and I would like to understand if it is just a defect or if I might have broken it so that I can avoid making the same mistake twice and break the second one as well. Let's call the first one I bought R1, and the second R2.

Here are the problems I have observed.

I first tried to use R1 as a NAS with an external self-powered USB hard drive. It seemed to work but often it froze after one day or two. I first thought the problem might be my USB power adapter: I am not an expert but I thought that maybe the power adapter does not provide a constant voltage and this could cause R1 to freeze. So I plugged R1 on my router, which has two USB ports, and used the router as a power supply. In this way R1 became much more stable. Before writing this question I tested R1 with my USB power adapter again and R1 would only run for a few minutes after which it spontaneously rebooted while it was in the middle of an apt-get update.

When I bought R2, I noticed that it does not have any problems with the USB power adapter. So, my first question is: can it be that R1 is more sensitive than R2 to changes in voltage and my router provides a more stable voltage than my USB power adapter? Does this mean that R1 has a defect or is damaged in some way?

Yesterday I noticed further differences in behaviour between the two. I wanted to attach two external USB drives to R1. The setting:

  • R1 powered by the router
  • Ethernet cable attached to R1
  • Self-powered USB hub attached to R1
  • Two external USB drives attached to the hub.
  • One USB-Ethernet adaper attached directly to R1 I knew that the hub and the drives work because I had tested this combination on my PC already.

I powered up R1 and logged in via ssh (no screen attached) but it could not detect the drives. I was getting error messages like hub_port_status failed (err = -71).

So I tried a different combination:

  • R2 powered by the USB power adapter
  • Ethernet cable attached to R2
  • The self-powered USB hub attached to R2
  • The two external USB drives attached to the hub
  • A USB keyboard attached directly to R2
  • An monitor attached to R2 with an HDMI cable

This time everything worked perfectly: after booting the drives were recognized and I was able to mount them.

I kept this setting and only replaced R2 with R1 (I also used the same SD card). The result was that R1 would not complete the boot: it kept rebooting again and again. I removed the USB hub and it completed the boot. After the boot completed, I attached the USB hub to it and it immediately rebooted after printing error messages which I couldn't read (too fast). After some experimentation removing one device at a time it could not even recognize the USB keyboard.

So I thought it must be a problem with the USB components. After reading this article I booted with only the Ethernet cable attached, logged in via ssh, and tried to perform a sudo apt-get update && sudo apt-get upgrade. Before the update was completed, R1 just rebooted. I tried it again, it rebooted again before completing sudo apt-get update. I attached it back to the router: it booted and I could complete the upgrade.

So the second question: besides the problem with the USB power adapter, is the USB component also broken? Can it be that I have done something, like plugging in the wrong device, that caused the problem? I do not have a problem buying a new Raspi if a got a bad one by chance, but I would prefer not to break them one after another by plugging the wrong devices onto them.

EDIT

I have just found this question. Can the polyfuse explain the behaviour I observed? I still would not understand why plugging in the external hub would cause a current overload on R1 and not on R2.

EDIT 2

I am a bit embarrassed to tell about the solution to my problem, but I want to post it because I find it instructive. During all my tests in which everything seemed to point at R1 as the defective component I totally overlooked that ... I was always testing R1 with one USB-power cable and R2 with another cable. So I switched the cables and now R2 does not work and R1 works perfectly. So: one USB-cable is bad.

The old principle of always switching one piece at a time.

  • This is an interesting story, but unlikely to help anyone solve your problem. Describe the status of the lights and/or measure the voltage. – Milliways Nov 15 '16 at 22:19
  • @Milliways: Thanks for the feedback. Do you mean the voltage of the USB power adapter? I will have to go and buy a voltmeter to do that. I can repeat the tests and look at the lights on the Raspberry Pi though. – Giorgio Nov 16 '16 at 10:06

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