On Sun, 28 May 2006 08:41:04 -0400, Jack Erbes
wrote:

The USB specs say that a USB port can provide 500mA for the device that
is plugged into it. If the advertising says that the drive can take its
power from the USB port it should mean that the drive draws less than
500mA at max draw.

Thanks, I didn't remember the exact number. I haven't checked, but
I'd bet the IBM drive I'm using now is more like 1 amp. I have a
Fujitsu drive that is 550mA; I should give it a shot.

The engineering on the laptop should be that the conductors for each
port are adequate to the rated max and that the USB bus is adequate to
all the ports drawing the max (four ports, would be 2A).

Yes, it should be that way. I'd been thinking the problem was that
some ports didn't meet the spec. Now I wonder if it isn't that some
ports exceed the spec. Those work for me, the ones that merely meet
the spec don't.

In the past, when using an unpowered external USB hub with a couple of
devices, I've gotten a Windows message saying that I'm exceeding the
power availabilty of the port. I don't know exactly how that message
gets triggered, but I've never seen it when using this drive.


If your drive works on some laptops but not on others, it must be
monitoring the available power or the load on the USB bus and electing
not to participate. I guess it could do that by measuring the voltage
drop on the USB bus?

Perhaps, but I didn't supply all the details. The drive spins up but
makes a clicking noise louder than a normal seek. It might be trying
to calibrate the heads or returning to rest position repeatedly. I
think it's actually trying to participate, but failing.

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