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Anecdote: Troubleshooting Woes
One of the subsystems I work on has Raspberry Pi 3, custom PCB that powers it (among other things) via IDC cable and external USB harddrive.

I provisioned many of these before, but this time HDD did not show up. Unmounted it, hooked it up to my Windows PC. USB to SATA adapter installed, but did not come up.

So I have faulty HDD, nothing uncommon. Grabbed second one. Also does not show up, second bad?

Obviously, first thing I considered was amount of power so added max_usb_current=1 at the end of /boot/config.txt, that should provide 1.2A. Also switched 24W power supply for beefier 60W one.

Grabbed second enclosure and third HDD and connected to my PC. Everything is fine. My USB power widget shows 0.25A. Then tested first and second HDD with second enclosure, all initialize and work on Windows. So it seems USB to SATA enclosure is defective. Just for the piece of mind connected first HDD to "bad" first enclosure before throwing it out and plugged to my PC. Works! WTF?!

Connected third HDD with second enclosure, combo that worked fine on Windows, back to Raspberry. Nothing.

Thinking there is some power drop that disturbs HDD, connected oscilloscope to the PCB jumper bridging 5V to Raspbery. As the probe is clamped to it and leaning on other part, it slowly slides the jumper up. Hmm, jumper is loose.

Replaced the jumper for another one, hooked up the probe. All power solid, no drops. Raspberry booted up and "bad" harddrive showed as expected.

Yes, this thing. The loose contact caused power failure to HDD that made it inoperable even on two Windows PCs until reinitialized in second USB/SATA enclosure

[Image: max6675_pcb.jpg]
Hi Roman,
Thorough testing and you found the solution. Thanks for sharing your adventure.
Bob D

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