USB devices are designed to be plugged in and unplugged without having to power down the computer they're attached to. This includes hardware devices like scanners or cameras, but also removable drives (whether actual hard drives or the flash-memory "keychain" variety).
Sometimes after a USB device is repeatedly inserted or removed -- or when a given USB device is unplugged once without being stopped by the user first -- the port it's plugged into may stop responding. If the port in question is on a USB hub, then all USB devices on the USB hub may stop responding as well.
Fixing USB issues
The problem can sometimes extend system-wide. It's been reported (courtesy of the JSI FAQ) that unplugging a USB 2.0 printer on Windows XP can cause other external USB 2.0 hubs to also stop working. Unplugging the USB hub usually won't fix the problem, either, since the problem stems from the USB controller itself. In such a case, one possible fix is to start the Device Manager, right-click on the device tree and select Scan for New Hardware to force a bus reset.
If this doesn't work, then the variety of USB controller in use on your system may have a bug that sometimes causes USB devices to become unresponsive when other USB devices are removed and reattached. One known way to work around this problem entirely is to disable the "selective suspend" feature of USB devices, which allows them to be individually powered down when not in use. Most of the time selective suspend isn't a critical feature, so it can be disabled system-wide with few ill effects.
To disable selective suspend on all USB devices, do the following:
- Open the Registry on the affected machine and navigate to the key HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\USB.
- Edit or create a DWORD value named DisableSelectiveSuspend and set it to a decimal value of 1.
- Reboot the computer.
Note that when you put this into effect, the "Allow the computer to turn off this device to save power" check box is no longer available on the Power Management tab for USB hubs. I've tried this on systems with a fair number of different USB-attached devices -- camera docks, scanners, hubs, printers -- and haven't noticed any side effects.
About the Author:
Serdar Yegulalp is editor of The Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators -- and please share your thoughts as well!
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