Martin Gregorie  wrote:
> On Fri, 01 Jan 2021 23:32:43 +0000, Nikolaj Lazic wrote:
>
>> Dana Fri, 1 Jan 2021 20:53:25 -0000 (UTC), bob prohaska
>>  napis'o:
>>> Just tried using a couple of USB card adapters with my 8GB Pi4.
>>> Both cause immediate loss of USB hard disk access. Unplugging the
>>> offending adapter does not restore normal operation. Power cycling puts
>>> matters right without apparent ill effect.
>>
>> Not enough power?
>
> From your description it sounds as if you're booting with the adapters
> disconnected and the hangup occurs when one or both are connected to the
> Pi. Is that what happened?
>
One adapter at a time, yes.

> Do they work correctly when used together on another computer?
>
They did when plugged into Pi3's, both under FreeBSD and RaspiOS

> Have you got a multimeter, osscilloscope or any other way to check the
> Pi's supply voltage?
>
It never crossed my mind that power might be an issue.
The power supply is a 3.5 amp unit from CanaKit. I suppose it's
possible I'm on the edge, but you'd think that would cause boot
troubles. I'll cobble up some test leads and check. No 'scope,
alas, but I do have a voltmeter. That limits the utility of
the measurement for transients. Hard to believe the puny little
adapter could make trouble while spinning up the disk is ok.
But it wouldn't be the first power supply to take sick on me.

> -------
> I tend to keep a Pi power cable modified so I can measure the voltage
> across it. I run the cable through a small plastic box with terminals on
> the lid, the terminals are type used on oscilloscopes or multimeters for
> connecting test probes. These are worth using because they're difficult
> to short accidentally. The terminals on the box are connected to the red
> and black wires in the power cable. So that putting a multimeter or
> 'scope across them lets you monitor the supply voltage going into the Pi.
> I sometimes put two terminals in series on the red cable with an on/off
> switch on the bit of wire between the 'red' terminals: this makes current
> measurement easy: turn the switch off and connect a meter on the two 'red
> terminals to measure current. Don't wanna measure current? disconnect
> meter from red terminals and turn the switch on. You mightn't use this
> gadget very often, but boy is it useful to have one when you suddenly
> need to check voltage or current in a USB terminated cable.

My habit has been to probe the GPIO pins. I hope the problem doesn't
become common enough to warrant a breakout box. Still it's a good idea.

Thanks for writing!

bob prohaska

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