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When a Powered USB Hub Is Not Enough

One of the questions I received recently was a surprise… Gary
Karasick, who subscribes to my newsletter, wrote to ask me about a
problem he was having…with his iMac:

I am having difficulties maintaining connectivity with y new
iMac. I have many external USB devices and instead of reaching
around the back of my iMac, I have used a powered USB 7 Port hub.
Sometimes I get a machine from my machine saying there isn’t enough
power to support all of the devices. How can this be if I am using
a hub with an external power source? What can I do so that I can
continue getting the benefit of the easily accessible hub?

I wrote back to Gary to say that I don’t use iMac or any Apple Mac
products, so I really could not answer questions specific to the Mac.

However, this problem wasn’t directly an iMac issue…it was a
problem with an external USB hub.

I suspected that his problem was that either his external power
supply for the hub didn’t provide adequate power to it, or that he
was connecting too much load on the powered USB hub.

A lot of the time, we connect a USB cable from an external powered
device, such as a printer, scanner or external hard drive to our
computer. The device gets its power from its connection to the
building’s power system (whether you call that “a wall socket” or
“the power mains.” Either way, it’s using the USB connection to
communicate data, but not to power the device.

However, USB connections are also capable of providing power. When
you connect a USB cable or a USB hub to one of the USB connectors
built into your computer, you’re making a connection that can
provide power as well as send and receive signals.

Examples of devices that are powered by your USB connections are
keyboards, mice, flash drives, and some portable external drives.
Sometimes, you’ll even find external devices that make not one, but
two connections, to your computer in order to get enough power to
drive them. External CD/DVD writer/players for notebooks and
netbooks are tiny things, but it takes plenty of power to drive the
electronics and power the motors to rotate the disks andn move the
read & write heads.

The problem comes when you need more power than the computer or hub
can provide. The USB 2.0 specification sets a maximum load (in
amps) that a device is allowed to take from the connection. The
computer’s hardware and the USB driver are set to monitor the load
— and to protect the hardware if the load is too much.

I have that problem sometimes with my Dell Inspiron 8600 and my
portable external hard drive (a Western Digital Passport 160GB
model). Sometimes, when I plug the drive in, the Dell (or Windows
XP) will trip the power to the USB port and give me a popup warning
that the load was too much. I’ve even had it do that while I was in
the middle of reading files.

I think this particular problem is specific to my I8600, as the same external drive has not caused the probem on any other computer.

You can have this problem when you connect too many unpowered
devices (that is, devices that are going to get their power from a
USB hub) to a USB hub and connect the hub to one of your computer’s
USB ports.

The normal fix is to get a powered USB hub. With this kind of hub,
you get a little transformer pack that you plug into your
electrical outlet and then you plug the connector from the
transformer into the USB hub.

So, Gary has the problem, even though he’s using a powered hub.

It could be that the transformer isn’t working, either having gone
bad or perhaps he’s got it connected to a power controller that
he’s managed to switch off.

It probably hasn’t gone bad. If he’s turned off the power, he’s
probably figured that out and turned it back on.

More likely, he’s plugged too many devices that want too much power
from the hub — and that want more than his hub can take from the
transformer. We’re back to the maximum load problem.

Fortunately, the feature that makes USB hubs even possible is that
the USB specification allows for daisy-chaining of USB devices to
other USB devices.

Gary’s fix is probably to get another powered USB hub, connect its
power to it and connect it to the powered USB hub he already has.
Then, he could split his USB devices between the two powered USBB


  1. My two hubs are powered, and though I don’t get error messages, often one or more of my peripherals (kybd, mouse or powered external drive) just stop working. I test the failed unit by plugging it into my second laptop USB connector and they work fine. I agree with your daisy-chain solution, and I’m beginning to think that I shouldn’t be buying 7-connector powered hubs but rather smaller hubs and more of them, so I don’t overload any.

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