Who Has Control over a USB Device?

Only one computer — host or guest — can have control of a USB device at any one time.

When you connect a device to a virtual machine, it is "unplugged" from the host or from the virtual machine that previously had control of the device. When you disconnect a device from a virtual machine, it is "plugged in" to the host.

Caution: You need to take a special step to disconnect USB network and storage devices from the host. There is a system tray icon called Eject Hardware on Windows 2000 and Safely Remove Hardware on Windows Server 2003. Use this icon to disconnect the device from the host before connecting it to a virtual machine.

Note: When you connect a USB network or storage device in a virtual machine, you may see a message on your host that says the device can be removed safely. This is normal behavior, and you can simply dismiss the dialog box. However, do not remove the device from your physical computer. GSX Server automatically transfers control of the device to the virtual machine.

Under some circumstances, if a USB storage device is in use on the host (for example, one or more files stored on the device are open on the host), an error appears in the virtual machine when you try to connect to the device. You must let the host complete its operation or close any application connected to the device on the host, then connect to the device in the virtual machine again.

On Linux hosts, guest operating systems can use devices that are not already in use by the host — that is, devices that are not claimed by a host operating system driver.

If your device is in use by the host and you try to connect it to the guest using the VM > Removable Devices menu, a dialog box appears, informing you that there is a problem connecting to the device.

To disconnect the device from the host, you must unload the device driver. You can unload the driver manually as root (su -) using the rmmod command. Or, if the driver was automatically loaded by hotplug, you can disable it in the hotplug configuration files in the /etc/hotplug directory. See your Linux distribution's documentation for details on editing these configuration files.

A related issue sometimes affects devices that rely on automatic connection (as PDAs often do).

If you have successfully used autoconnection to connect the device to your virtual machine, then experience problems with the connection to the device, take the following steps:

1. Disconnect and reconnect the device. You can either unplug it physically, then plug it back in or use the VM > Removable Devices menu to disconnect it and reconnect it.

2. If you see a dialog box warning that the device is in use, disable it in the hotplug configuration files in the /etc/hotplug directory.