XPost: comp.os.ms-windows.misc, alt.comp.os.windows-7, alt.windows7.general   
   From: me@privacy.net   
      
   On 15/08/2025 22:03, Paul wrote:   
   > USB drives, can be operated from bus power, or from a separate power source.   
   > Bus power is limited, and a lack of bus power causes some USB hard drives   
   > to fail to appear when plugged in. A USB power meter, in series with   
   > the USB connector at the host, will show whether power is present   
   > for any period of time. A Polyfuse can open on the motherboard and   
   > remove power from the port.   
   >   
   > USB sticks, also draw bus power, but some of the slower bus standard USB   
   > sticks should work even when bus power is weak. My oldest USB stick is   
   > a 1GB one, and that is unlikely to draw a lot of power.   
   >   
   > The power class declaration on a USB item, is unlikely to be exactly   
   > equal to the actual power draw. For example, a device rated at "98mA",   
   > that is a fake rating, intended to be "short of the 100mA value". The device   
   > might draw 5mA, for some types of USB items. There have been cases, where   
   > a 5V @ 500mA device (Alcatel Frog Modem), a power measurement showed   
   > it was drawing 530mA. That's OK, as the Polyfuse is set to higher than that.   
   > Only a laptop with a silicon fuse, would have trouble with an Alcatel   
   > Frog Modem. A desktop should be fine. That device gets special mention,   
   > because a few people did have fuse-related outages with it. Other   
   > people were fine.   
   >   
   > A 2.5" hard drive, some spec sheets don't show the spinup current,   
   > but the current is on the order of 1000mA to 1100mA or so. When sitting   
   > on the USB2 bus then, that's a wee bit of an overload. The Polyfuse   
   > may be set at 1100mA on a "stack-of-two" with black plastic tabs.   
   > The USB3 ports, with their 900mA rating, the shared fuse on those   
   > is over 2 amps. Not all PCs have a USB3 port and the more-generous fuse.   
   >   
   > In any case, it's not really a good idea, to occupy absolutely all USB   
   > ports on the back of the computer, with USB hard drives (spinning kind).   
   > My Seasonic supply, the label only offers "5VSB @ 2.5 amps", and   
   > that is enough to keep five drives spinning, but it can only guarantee   
   > that two drives spin up when power is applied. Using one or two   
   > drives, would be as much as I would want to plug in. While at one   
   > time, the USB ports ran off the more generous +5V rail, the ports   
   > today run off +5VSB, and there is no jumper option to change that.   
   > The +5VSB is a weak rail, for operating a lot of toys.   
      
      
   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.   
      
   --- SoupGate-Win32 v1.05   
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