Re: Losing connection with USB drives

On 2025/8/16 15:2:31, NY wrote:

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> I have had problems with bus-powered USB devices that are powered by a 
> USB hub rather than by the computer.
> 
> The general advice with the Raspberry Pi (so not Win 7!) is that 
> bus-powered spinning hard disks should be powered by a USB hub with its 
> own power-supply rather than from the Pi itself. This worked find for 
> the Pi 3, but when I got a Pi 4, I found that it failed to boot (not 
> even any output on HDMI so no diagnostic message) if the powered hub had 
> power. If the hub was unpowered, the Pi booted but (as expected) failed 
> to see the external drive. If I booted the Pi and then turned on the USB 
> hub, I could manually mount the drive.
> 
> Some people suggested that there was a conflict between the Pi's power 
> through the USB port and the external power to the hub. I made up a USB 
> cable with the +5V line cut. This made no difference to the booting 
> problem. It did still allow the drive to communicate as long as the 
> power to the hub was off at boot time and only later turned on.
> 
> I had to resort to a powered caddy and no powered USB hub. That works 
> flawlessly.
> 
> So powered USB devices can be a pain.

I have had PCs that wouldn't boot if a USB memory stick was plugged in.
I can't remember whether it was any memory stick in any port, or just
certain sticks, or certain ports. It would have been, I'm pretty sure,
either Windows 7 or XP (USB under Windows 9x was pretty flaky anyway). I
haven't had that problem with this W10-64 machine, but possibly only
because I've never tried booting it with a stick plugged in so far.

Yours is interesting: it saw the hub if unpowered. Presumably it was a
hub that could be either externally powered or bus powered (but of
course couldn't power a high-load peripheral if only bus-powered). My
guess was that its internal processor (or registers, or whatever) went
to - or at least ended up in - a different state if external power was
applied before communication commenced.
-- 
J. P. Gilliver. UMRA: 1960/<1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

Self Test for Paranoia:  You know you have it when you can't think of
anything that's your own fault.
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