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I have an external USB hard drive which works perfectly most of the time but sometimes when I plug it in nothing happens.

Nothing is outputted to dmesg when this happens. Rebooting always fixes the problem.

How to I go about diagnosing this if there is nothing in dmesg?

Linux version 3.8.0-31-generic on Ubuntu 13.04

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It sounds like a cable problem.. how old is the usb hard drive? I've had similar issues, and the problem was the usb cable, one wire was cut and needed to be changed. – medigeek Oct 12 '13 at 18:20

It seems that there is not always enough power from USB to start hard drive. Try using additional power supply for the drive, or use USB Y-Cable to provide up to 1A to the drive.

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