Devices disconnecting? It's about the hub, bub

big usb hub

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Reader Micha Warwick has a problem with power. He writes:

I recently purchased a powered 10-port USB hub because I have a lot of peripherals. I’ve used nearly all the ports for external devices—a web cam, some external drives, and so on. When I plug in my camera I see a message that one of my hard drives has been disconnected even though I haven't touched the cable or powered down the drive. Is the hub or drive broken?

No. In fact, the hub is operating exactly as it's supposed to.

USB hubs like this are designed to provide only so much power—say 250mA (Milliamps) on average to 10 devices. As long as the power requirements of all the devices you jack into the hub fall under that aggregate limit, all 10 devices should be recognized. However, if you exceed the hub’s limits—you’ve plugged in several devices that draw 500mA, for example—something’s got to give. And generally, what gives is the disappearance of one of the connected devices, as evidenced by the disconnection message you’ve seen.

And how can you tell which devices are pulling a lot of power? Hold down the Option key, click on the Apple menu, and choose System Information. In the resulting System Information app select USB under the Hardware heading. In the list of USB devices on the right, select the hub you’ve connected to your Mac. At the bottom of the window you’ll see a list of all the connected USB devices.

usb current required

The Current Required field will tell you what's sucking up your hub's power.

In this list look for the Current Required (mA) entry for each device. The number appearing here doesn’t tell you how much power the device is currently drawing but rather how much it's requested that the hub make available to it.

Something like a printer may require very little power whereas a web cam or media card reader may want the full 500mA. (Some Apple devices such as iPhones and iPads require more than 500mA at 5 volts so that they can fast charge.) Run through the list and do the math. If your list of devices is pulling too much power from the hub, consider moving some of them to a different USB port on your Mac.

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